Given any polygon with n sides, the sum of its interior angles is
[tex] (n-2)\times 180^\circ [/tex]
Your polygon has 5 sides, so its interior angles sum up to
[tex] (5-2)\times 180^\circ = 3\times 180^\circ = 540^\circ [/tex]
Which means that
[tex] 100+102+108+121+x = 540 \iff 431+x = 540 \iff x = 540 - 431 = 109[/tex]
