What expression represents the measure of angle x?

The isosceles triangle has two sides congruent which mean they are equal. If two sides are equal, it also means that two angles are equal. 

For triangle XYZ, we have angles shown below:
∠X + ∠Y + ∠Z = 180°

If ∠Y = a°, then we can have the expression for ∠X such as shown below:
∠X + a° + ∠Z =180°
∠X= 180° - a° - ∠Z

If ∠Z = ∠Y, then we have the final expression:
∠X = 180° -2a°