The step where the error occurred is Step 4. After step 3, 34x should've been added to both sides of the equation. Instead of -17x, Step 4 should say 17x, because -17+34=17. Somehow, the whole number in Step 4 also changes from 40 to 14. Nothing changed it, it should still be 40. Step 4 should be: 17x=40.
The answer is 15 inches. because the opposite side to 10 inches is the same we can subtract 20 from 50. that leaves us with 30. divide that by two and you get 15.