Slope-intercept form:
y = mx + b ["m" is the slope, "b" is the y-intercept (the y value when x = 0)]
So all you need to do is get "y" by itself:
9x - 7y = -7 Subtract 9x on both sides
9x - 9x - 7y = -7 - 9x
-7y = -7 - 9x Divide -7 on both sides

or
(I just rearranged it)