Ineed solving for x. the answer is 135° but i have no idea how to get it

First, to find the measure of the interior angle, you have to find the su of all the angles:

1080 degrees

Then you take that and divide it by the number of sides on the polygon, so in this case, 8 sides, and divide the total, 1080 degrees, and divide it by 8:

1080/8= 135

And, to solve it the other way around, if the interior angle isnt the variable, you multiply the measure of the interior angle by the nuber of sides on the polygon:

135x8= 1080