Step-by-step explanation:
Horizontal components:
10 N + 12 cos (-100) = + 7.916 N
Vertical components
0 + 12 sin (-100) = - 11.818 N
To have equilibrium, the third force must be of the same MAGNITUDE and OPPOSITE direction
so Horizontal = - 7.916 N
Vertical = + 11.818 N
Magnitude = sqrt ( -7.916^2 + 11.818^2 ) = 14.22 N
at angle arctan (- 11.818/7.916 ) = 123.8 degrees