Answer:
x = 12
Explanation:
Here we go!
What we know:
So 7x and 8x are part of a same side interior angle. meaning, they add up to 180°!

Combine like terms

Divide 15 from both sides

Let's check our work:
plug in x
7(12) + 8(12) = 180
multiply
84 + 96 = 180
add
180 = 180