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1. If I have the gradient and a point the line passes through then how do I work out where the line crosses the y axis?
2. the equation of a line is y=mx+c
the gradient will give you the m, so for example if the gradient is 4 your equation is now y=4x+c
then you input the point into the equation so for example if you has (2,3) your equation would now be 3=4x2+c
you then solve this to find c --> c=3-(4x2) ---> c=-5
because c is the y intersect you now know the line crosses the y axis at -5
3. (Original post by libbydavis)
If I have the gradient and a point the line passes through then how do I work out where the line crosses the y axis?
Every straight line has the equation: y = mx + c
(where m is the gradient and c is the y-intercept)

You have been given the gradient so substitute this value in for m.

Using the point that they have given you, substitute in the values of y and x, and then rearrange the equation to work out c.

Form the equation for your line using y = m(given)x + c(what you worked out)

When a line crosses the y axis, x will always be 0. So substitute x = 0 into the equation you made and you will be able to find c.

For example:

"a line with a gradient of 3 passes through the point (5,7). Find out its y-intercept" (I just made this up lol)

y = mx + c
y = 3x + c
7 = 15 + c
c = -8
y = 3x - 8

when x = 0:
y = 0 - 8
y = -8

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