To find ∠C you don't even need 90° unless you would want to check if you are correct. to find ∠C you would just look for the whole triangle, so basically A + B + C = 180.
80 + 30 = 110
180 - 110 = 70
∠C = 70
Then you would take 70° and subtract it to 90° since it is a right angle combined with x and you would get your answer which is 20°.
