The diagonals of a rhombus bisect each other at right angles. So a rhombus can be viewed as the union of 4 congruent right triangles, each with legs (1/2)*18 = 9 and (1/2) *24 = 12. See attachment. This gives a hypotenuse length of 15 so the Perimeter = 4 *c = 60.
You would take the amount of increase and divide it by the amount of total cost over the time period of certain months. Then you would multiply that number by the percentage of increase gained, and subtract 96.12 to get your answer.