You need to sum the similar terms. Remember that similar terms are those that have the same letter and the same exponents for the same letters. You can order the expression to make the work easier: 4m + 9 + 5m - 12 = 42 => 4m + 5m + 9 - 12 = 42 => 9m - 3 = 42. Now sum 3 to both sides to obtain 9m -3 + 3 = 42 + 3 => 9m = 45. Now divide both sides by 9 to obtain: m = 45 / 9 = 5.. And the answer is m = 5.
