Answer:
Select these:
x
-x
40x
-30x
In all of these terms, a number is being multiplying by x (and x only) just like in the question! That's how to tell.
For the "x" option, remember that an x alone equates to 1x. So, 1 is being multiplied by x.
For the "-x" option, remember that an -x alone equates to -1x. So, -1 is being multiplied by x.
