This is actually quite simple using the exterior angle theory
"The exterior angle theorem states that the measure of an exterior angle is equal to the sum of the measures of the two remote interior angles of the triangle"
In this case the exterior angle is 120 degrees, basically what the theorem states is that it the angle that you see outside the triangle equals the angle measurements of the 2 angles opposite of it, so just create an equation because you know 2 of the variables
