There are two methods to find the measure of the third interior angle of the triangle.
The first is to set the interior angle values equal to 180 and solve for the unknown angle.
180 = 35 + 90 + m∠C
Collect like terms.
180 = 125 + m∠C
Subtract 125 from both sides.
55 = m∠C
The second method is to simply subtract the two known interior angle values from the known sum of all the interior angle values.
180 - 35 - 90 = m∠C
180 - 35 - 90 = 55
Both render the same result, although the second is far faster.
Answer:
m∠C = 55°