You didn't go wrong but rather didn't complete the whole equation. The problem you made appears to be that you cancelled out sin(x) which you can't do because sin(x) is potentially 0. I've outlined the method using factorising below

From the domain restrictions

Multiply by cos(x) and factorise

or

This is because cos(x) is symmetrical in the line so we can do where 180-70.5 is the distance the first point is away from 180