First find out how many questions he got right setting up an equation.
1)3x=84.
The variable 'x' is the answers he got correct. The next step would be to divide by 3 to get the x alone. Remember, whatever you do to one side is done to the other so divide 84 by three as well
2) 3x÷3 = x.
84÷3=28.
which leads you to x=28.
Then, since he got 7 wrong, you would add the 7 to the 28 he got correct to get the final answer.
3) 28+7=35
another way you can do this is multiply numbers by three until you get to 84. The number you multiplied by is the amount he got correct, then add the correct and wrong ones together to get 35
