Issue #40738 Hints showing incorrect result.

In the subtraction problem, the math was not being done for the last
step of the hints, resulting in an incorrect value.
Focus hints around combinatorics.

Hints focus on binomial theorem with combinatorics. Use Pascal's
triangle as a side note. Fix a bug in computations when x coefficient >
1.
Exponent Equations implemented

Exponent Equations implemented from the Trello board. Example: 3^(2x) =
81^(x-2)