Try this solution (this is not the shortest way):
1. The rule: s=180(n-2), where s - the sum of the internal angles of a polygon, n - the number of the internal angles of the polygon.
2. According to the rule described above, the sum of the internal angles of the given polygon is: s=180*(6-2)=720°;
3. Using the calculated sum it is possible to determine x:
x=720-(90*2+135*3)=135°.
Answer: 135°
