What you should do in this case is to see the terms that are repeated in the expression.
We have then that the terms are a and b.
Making a common factor we have:
a * b (a * b -7) +10
Answer:
The equivalent expression is:
a * b (a * b -7) +10
Answer: C, 12 units on edge
Step-by-step explanation:

The last line is in vertex form, and can see the vertex is located at (2,1).
The greatest common factor of the expressions 60x and 84 is 12 because it can divide both 60 and 84 giving us the answer of 5 and 7, respectively. Factoring out 12 from the terms of the given expression will give us the answer of,
60x - 84 = 12(5x - 7)