This puzzle looks like a kakuro (the logic puzzle type in which the solver must place a digit from 1 to 9 in each square so that there are no repeated digits in any across or down entry, and the clue in front of the entry gives the sum of that entry’s digits). However, the clues are much too large for the kakuro to be solvable.
However, each of the large-number clues has factors that would be small enough to be valid clues. This discovery, the title reference to “stacking,” and the flavortext reference to two pages being stuck together (and to a lesser extent, the word “times” in the flavortext) should hopefully suggest that this puzzle is actually two kakuro puzzles combined, and each given clue is the product of the two clues from the two puzzles.
There is only one way to deconstruct the clues into two solvable puzzles; the solutions to these are shown below.
The letter equations appear to be asking you to perform arithmetic on the numbers in the solved puzzle . . . but in which solution? Well, the marked squares are in the combined grid (with product clues), and so the numbers that would go in that grid would logically be the products of the numbers from the two solution grids! That combined product solution is shown below.
Performing the arithmetic on the products in the lettered squares gives seven numbers from 1 to 26. Interpreting these numbers alphanumerically gives the answer to the puzzle, RIGHT ON.
12 + 9 - 3 = 18 = R
45 + 12 - 48 = 9 = I
16 + 72 - 81 = 7 = G
15 + 49 - 56 = 8 = H
36 + 8 - 24 = 20 = T
14 + 49 - 48 = 15 = O
6 + 24 - 16 = 14 = N