You just have to rearrange the formula.
So:
765 = 135 x m + 225
765 - 225 = 135 x m
540 = 135 x m
m = 540 / 135
m = 4
So four months.
The question is asking for a fourth of a third.
1/4 of 1/3
"of" = multiplying in math
1/4 x 1/3
= 1/12
That means it is cut in twelfths and it has 12 slices.
Answer= 12 slices
Hope this helps :)
ANSWER

EXPLANATION
The given expression is

To find an expression for d in terms of a, b, and c, means we want to make d the subject of the relation.
This means that, we need to isolate d on one side of the equation while all other terms are on the other side.
We subtract from both sides of the equation to obtain,

We now divide through by -1, to get,

Or

The topic is change of subject