The answer is C. You can make a rectangle by putting the diagonals through the center anywhere, but if you want a square, you have to put the two diagonals perpendicular to each other and join their ends. See the diagram.
Simple...
you have: 15x+2=17
Isolate the variable (x)-->>>>
15x+2=17
-2 -2
15x=15
x=1
To check it-->>>
15(1)+2=17
17=17
Thus, your answer.
That is 3 slots
12 in 1st slot
11 in 2nd
10 in 3rd
so that is 12*11*10 or 1320 ways
To find area so l x w x h