To find the correct equation for step four, we must rewrite step three:
-36 = 3d + 6
The next step in solving this problem is to isolate the variable, meaning we get rid of 6 on both sides by subtraction:
-36 - 6 = 3d + 6 - 6
-42 = 3d
Therefore, the correct equation for step four is -42 = 3d (A).