The given figure is a right triangle, and the sum of two acute angles other than the 90 degree is always 90.
So the equation will be :
Solve for the value of x :
x = 90 - 55
x = 35
Use sine function in finding the value of a :
Note that :
Where alpha is the angle equal to 55,
"a" is the opposite side
and
10 is the hypotenuse
Therefore, the correct answer is Option C.