If this is exercise #12 on your homework, then you are being instructed to solve for the unknown variable in the equation 3n - 10 = 12
Step 1) To isolate the unknown variable n, add 10 to both sides of the equation.
Now you have the equation 3n = 22
Step 2) To further isolate and subsequently solve for the unknown variable n, divide both sides of the equation by 3.
Step 3) Your solution is: n = 22/3 = 7 1/3 (the mixed number "seven-and-one-third")