Question

In the equation y=-3x as x increases what does y do

Answer 1

Because the slope is negative,  you would expect that y decreases and x increases. To check this, you can substitute values for x. At x=0, y=0. Increase x to 1, and y=-3.

Answer 2

If x increases then y will decrease

prove:
y= -3x

if x = -1, then y = -3(-1) = 3
if x = 0, then y = -3(0) = 0
if x = 1, then y = -3(1) = -3
if x = 2, then y = -3(2) = -6
if x = 3, then y = -3(3) = -9
and so on...