I am assuming that you will need to do Pythagorus Theorem to find the 3rd side length. If thats the case then you will need to do the following formula : a^2 + b^2 = √(c)
So 15^2 + 30^2 = √(c)
= 33.5 inches
The third angle would be 115 because 180 - 45 - 20 = 115
I hope this helped.
