$ 100- 25cos (20) = 76.5076 $

Force in member ob.

We divide the tributory of 75 N by this to get the cos(b-a)
75/ 76.5076 = 0.98029

Arccos (0.98029)= 11.3934

This  added to 20 is angle beta.

Beta = 20 + 11.3934 = 31.3934

And Omega = 180- 31.3934= 148.606

Sum of angle in a triangle =180°
Omega=180-(70+31.394)