Explanation:
I'm pretty sure you just use the law of cosines in this case.
c= sqrt(a^2+b^2﹣2ab (cos γ))
where a is a Side, b is a Side, γ is Gamma or the angle. Sqrt means square root
In your case, a is 20, b is 13, and γ is 93. Just plug it into the formula now :). I got 24.41751 ish.