Since it is shown that 2 of the sides of the triangle are congruent, we can assume that they have the same angle measures.
so to solve for the missing angle you can set up an equation equal to 180 since all triangles add up to 180 degrees.
21 + 21 + x = 180
42 + x = 180
x = 138
the value of the missing angle should be 138 degrees
hope this helped