What expression represents the measure of angle x?

An isosceles triangle is a triangle that has 2 equal sides and 2 equal angles. 

side XY = side XZ
∠Y = ∠Z
Since ∠Y = a°  then ∠Z = a°

Because interior angles of a triangle sum up to 180°, we simply need to find the difference between 180° and the 2 known angles to solve for the measure of angle X.

∠X = 180° - 2a°