Theorem
The measure of an exterior angle of a triangle equals the sum of measures of the remote interior angles.
Angle STV is an exterior angle of triangle SRT.
The two remote interior angles to exterior angle STV are angles S and R.
Since the triangle is isosceles, with sides ST and TR equal, angles S and R are equal, so they both measure 27 deg.
<STV = 27 + 27
<STV = 54