Let's use a formula in order to find how much the interior angles are supposed to add up to.
(n-2)×180; n is the number of angles.
(5-2)×180= 3×180=540
We need for the interior angles to add up to 540. Let's make an equation.
100+102+108+121+x=540
431+x=540; let's find x.
540-431= 109
So, X= 109°