To do this plug in 0 for the x and y when you are trying to find one value like this:
First I would plug in 0 for y:
0 = -7x +3
Now do anything to isolate x:
-3 = -7x
3/7 = x
Now plug in 0 for x to find the y
y = -7(0) + 3
y = 3
