This can seem a bit tricky at first trying to remember when sin(x)=cos(x). Think of your special triangles. Remember the 45-45-90 one? It has legs of 1 and 1 and a hypotenuse of square root of 2. The sine and cosine are the same here. So at 45 deg. or , sin(x)=cos(x). But like most trig functions, it's going to repeat itself with more than one angle. What other angles in the domain is this true?
You can also think of it another way using that
EDIT: Hint - One of the main trig functions is negative in the 2nd, 3rd, and 4th quadrant. How does restrict sine and cosine being equal, meaning which quadrants require sine and cosine to be opposite signs?