Describe and correct the error in solving these equations

16. If d equals 21 then 21 goes over the two, to make it 21/2-5=8. then replace the d with 5 the make the problem 21-5=16, which is right. put 5 over 1 and subtract 21 over 2 and 5 over 1 but make equivalent denominators first, then subtract across on the top, and simplify and see if it equals 8.

17. Substitute 3 for g and make the equation 16=4 3-4, multiply 4 and 3 first then subtract 4 from it, which is 9, so it does not equal 16. replace 3 for g, which makes the equation 12=4 3, the answer is 12, so it does equal 12. 

The second step on the test was the move he was supposed to