If m∠ACF = (a + 20)°, and m∠AGF = (2a + 10)°, find

m∠AGF.
A. 150°
B. 170°
C. 20°
D. 30°

First let's put both into equivalent forms by solving the second equation for y.
8x - 4y = 12
-4y = -8y + 12
y = 2y -3

We now have two equations:
y = mx - 6
y = 2x - 3

Recall that parallel lines have the same slope, and the slope is "m" for an equation in slope-intercept form y=mx+b.
The slope in y = 2x - 3 is 2, so for the equation y = mx -6 to be parallel, it must also have a slope of 2.
Therefore, m = 2.