Which of the following is a decreasing function?
a. y = 7 • 3x
b. y = 7 • 0.3x
c. y = −7 • 0.3x
d. y = 0.7 • 3x

C. y=-7(0.3x)= -2.1x
a function is decreasing if the slope(or number in front of x) is negative

B and D are the same equation by associativity of multiplication, so assuming you have to select only one equation, those two are out. The first one (A) is y=21x, which as x increases, y increases. The third one (C) is y=-2.1x, which as x increase, y decreases (gets more negative). You can check by just seeing what happens when x = 0 and, say, x = 1. If x = 0 then y = 0. If x = 1 then y = -2.1. It decreases. This isn't a proof in itself, but you can definitely see that the other three equations are increasing at some point, which means that they can't be decreasing functions.