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.

Ibelieve it is

We’re gonna need a picture to refer to in order to figure it out. try reposting your question with the picture included.