I tried figuring it out on my own, but I don't get it..

Here's one way of doing this

Cos (90 - x) = √3/2

Notice that

Cos (30°) = √3/2

So.....x must be = 60°

So.....sin (60°) = √3 / 2 !!!!!

Does that make sense, RP???

Yes, it does make sense. Thank you!