Can someone me simplify this answer i keep getting it wrong?

The answer is -8

I found my answer by using desmos. It is the best app for graphing. I wrote the equation and then I looked for the point 18 on the y axis and then I looked where it was on the x axis

that is how I found my answer.

glide reflection.

its mainly because the pattern will be infinite and will never end. it will look like a gilder or like a filpbook when set to motion.

i think it would be 42cm