simultaneous equation help

The question is

3x-4y=4
9x-7y=22

so I multiplied the first equation by 7 and second by 4 so they both give the same value for the middle bit which is 28.

After that I don't understand.

The question is

3x-4y=4
9x-7y=22

so I multiplied the first equation by 7 and second by 4 so they both give the same value for the middle bit which is 28.

After that I don't understand.

So you have -28y in the first equation and -28y in the second. Try taking (the new first equation) from (the new second equation).

(okay well someone else wrote an answer as well but i was writing mine and i don't want my work to go to waste so i'll just post it okay)

3x-4y=4
9x-7y=22

Well, the first thing you need to keep in mind is that, based on what is easiest, either the x's or y's should be comprised of the same number, except on negative and one positive. (or both the same if you'll subtract the equations instead)

In this case, it'd be easier to just multiply the first equation by -3. Saves time and smaller numbers.

So:

-9x+12y=-12
9x-7y=22

So, heres the thing, we add up the equations. (Why? Because logically speaking, if -9x+12y=-12 and 9x-7y=22, then -9x+9x+12y-7y=-12+22 )

So basically: (see spoiler)I added -9x+(+9x) to give 0. This was why we multiplied, so that we could eliminate one of the variables.

Then +12y+(-7y) to give +5y. Then, then last one is explanatory.

Now that you know y, substitute it in any of the original equations and get x.
Then test both x and y in the remaining equation to make sure it gives the same end result.

(p.s I know I did a slightly more complicated route, but that's what I was used to. I can't believe I never thought of the subtract method before. That's my whole life lived like this. What a life it was)

3x-4y=4 ---> 3x = 4 + 4y ----> x = 4/3 + 4y/3 ----> sub in x for 9x-7y=22 ----> 9(4/3 + 4y/3)-7y=22 ---> now you can work out y
9x-7y=22

What am I doing wrong in this question? :/

Posted from TSR Mobile

What am I doing wrong in this question? :/

Posted from TSR Mobile

I think it's because you forgot to multiply 18 by 5 lol

Oh lol thank you X

Posted from TSR Mobile

haha, no problem. we've all suffered from silly little mistakes :3

This seems wrong :/

Posted from TSR Mobile

You've probably figured this out by now, but you made a mistake adding 240 and -370. You accidentally did 240+370, not 240+(-370). It should be -130, not 610.

Remember to always keep the signs in check :3

(I got x=-2 btw)