Hello
Lets solve your equation step by step
42 * n + 12
Step 1: Subtract 12 from both sides. (The reason why we do this is because, 42 is multiplied by the variable (n). You need to subtract the number that is on its on.)
42n + 12 - 12 = 558 - 12
42n = 546
Step 2. Divide both sides by 42. (We divide both sides by 42 because you have to do the opposite. If this was division, we would multiply 42 on both sides)

546/42 = 13
Lets double check:
42 x 13 + 12 = 558?
42 x 13 = 546
546 + 12 = 558
So your answer is: n = 13
Hope this helps