[tex]m\operatorname{\angle}a=60[/tex]
1) As we can see there are two triangles, similar triangles. Similar triangles are proportional on their sides and congruent on their angles.
2) So, the angle a can be found by doing the triangle sum theorem
[tex]\begin{gathered} m\angle a+90+30=180 \\ m\angle a+120=180 \\ m\angle a=60 \end{gathered}[/tex]
