Exactly one value in the set {1, 6, 9, 18} is a solution of 5m − 8 = 3m + 10. Which one is it?

Gather like terms: 5m − 3m = 10 + 8 → 2m = 18 → m = 9. Check: 5(9) − 8 = 37 and 3(9) + 10 = 37 ✓. The value 1 comes from flipping a sign wrongly (2m = 10 − 8); 18 from forgetting the final division by 2; 6 from dividing 18 by the 3 in '3m' instead of by 2.