O<-Sun ------------------------- |90 O<-earth
If that's your situation, why not simply use the distances of Earth and Venus from the Sun, and calculate the angle using tangent like this,
tangent Sun - Venus = (distance Sun - Venus) / (distance Sun - Earth) ?
So if the distances were
Earth = 1 AU
Venus = 0.72 AU
0.72 / 1.00 = 0.72, take inverted tangent (tan^-1), and you'd get 38.8 degrees separation between Venus and the Sun from Earth's perspective (as per your scenario).
Tangent is the trig function that fits your bill, as it is TOA, meaning opposite side of the angle over adjacent side. You need to refresh your basic use of trig functions on right triangle and Pythagora's law, before proceeding.