Just do 28 times 3 and you will get 84 to check this just divide 84 to 28.....
we have given that 
where
represents the number of elements in C.
where we have 4 and 6 which have two elements.
so 
Using the Pythagorean theorem:
12^2 + x^2 = 15^2
144 + x^2 = 225
x^2 = 225 - 144
x^2 = 81
x = sqrt(81)
x = 9