The exterior angles of any polygon add to 360 degrees.
If we call the interior angles A,B,C,D,E that means
(180-A) + (180-B) + (180-C) + (180-D) + (180-E) = 360
5(180)-360 = A+B+C+D+E
A+B+C+D+E = 540 degrees
So we know the indicated interior angles add to 540.
90 + 148 + 3x + x+10 + 112 = 540
4x + 360 = 540
4x = 180
x = 45
Answer: Choice C: 45