Lets use the variables "a" and "g" to solve.
a = number of axles
g = number of engine blocks
If Jessie has 12 more axles than the number of engine blocks, this means we are adding (g + 12).
Now, we have the expression a = g + 12
Solve for g since we are finding the number of engine blocks Gus Installed.
a = g + 12
~Subtract 12 to both sides
a - 12 = g + 12 - 12
a - 12 = g
Therefore, the answer is [ a - 12 = g ]
Best of Luck!