You can see that 10x + 21 and 13x are equal to 90 degrees. So set those equal to each other. No combine the like terms in this case 10x and 13x. Now you have 23x+21=90 Subtract 21 from each side. That leaves you with 23x = 69. Now divide each side my the 23 and you get x = 3. If angle DEH Is 13(x) plug in the x which you got as 3) that gets you 39. Now do the same for angle FEH which gets you 51. Some people are visual learners so i added a picture.
