Which of the following lines goes downwards from left to right? a. y=2x+5 b. y=2x-6 c. y=2x d. y=-2x+20

"Goes downwards from left to right" is basically saying that the slope of this line is negative. We go downhill from left to right. 

y = 2x+5 has a positive slope because m = 2 is the slope (b = 5 is the y intercept)
So choice A is out

Similarly choice B is eliminated as well because the slope is also m = 2

Choice C is false as well. The slope here is m = 2 (think of y = 2x as y = 2x+0)

The only choice with a negative slope is choice D. The slope is m = -2 

This is why the answer is choice D

Side Note: I'm using slope intercept form y = mx+b where m is the slope and b is the y intercept

For the first question the answer is a, for the second c, for the third d, for the 4th b, for the 5th a