Question

What is the y-value of sin(x) when x = -90°?

Answer 1

This is a good time to know your unit circle, are you familiar with what that is? @aliivictoria

Answer 2

So sin(90) is what you are asking. Using your calculator, solve for sin(0), sin(45), sin(90), and these answers you should try to memorize =0)
Do the same with cos, tan. These 9 answers will come up over and over again in Trig and will help you a LOT.

Use the sin(90) you solve on Calculator and you will see it equals 1.
The sin of 90 Degrees is 1

Answer 3

I apologize, sin (-90) is your question.
sin(-90) = -1