Whatever you do to one side of the equation you must do to the other. If you’re new to it, before it becomes second nature it will be helpful to write out every step.
For example:
a + bc = d
If you want to mace c the subject say, first you can subtract a from each side
bc = d - a
You can substitute numbers in and convince yourself these two equations are still equivalent
Then you want to divide both sides by b
c = (d-a) / b
And this logic applies to much more complicated expressions as well.
If that’s still a bit confusing, it works because if you have an equation
2*4
This = 8
But if you divide everything by 2 for example you get
1*4 = 4 which is still a true equality.
Hope that makes sense