Okay so basically, these are vertical angles which means they will be equal to each other. Basically your equation is 7x + 13 = 48. Now it is time to solve for x. Subtract 13 from both sides. Which means subtract 13 from 13 and subtract 13 from 48. Now your new equation is
7x = 35 now divide 7 on both sides, which means divide 7x by 7 and divide 35 by 7. This means that x=5.
I hope I helped.