Question

What is the value of x?

Answer 1

Look up "sum of interior angles of a polygon" on Google.  There you will find a formula for predicting the number of interior angles that a given polygone will have.  This formula is  (n - 2) × 180°, where n represents the number of sides of the polygon in question.  Count these sides.  Substitute your value for n into this formula.  The interior angles of your polygon add up to the result of evaluating this formula.  Call this "sum."  Then:

"sum" = (the one unknown interior angle) + (the sum of the given (known)) interior angles).