Draw a triangle with sides a=45mm, b=45mm and c=60mm.
Draw a line from the midpoint of c to the opposite corner of the triangel (i.e. the height of the triangle).
The angle \(\gamma\) at that corner satisfies
\(\sin\frac{\gamma}{2} = \frac{60/2}{45}\)
because the height splits the triangle into two congruent right triangles.