Question

I have no idea where to start

Answer 1

Let me know if you want the answer, but what you need to do first is get the opposite angles.

They're on straight lines, so 180 minus that angle would equal the angle inside the triangle.

Then all inner angles if a triangle add up to 180° so 180 minus the two other angle would give you the final missing angle.

Do the straight line thing again and subtract 156 from it.