In the equation y = 4 x, if the value of x is increased by 2, what is the effect on the value of y ?
well, just plug it in and see what you get:
y = 4(x+2) = 4x+8
so, y increases by 8.
You could also have seen this by noting that the slope is 4, meaning that every time x increases by 1, y increases by 4.