Can you show me how you got x11.28 from x+9.72=21

Witch represents the inverse of the function f(x)=1/4x-12

Frank has a devised a formula for his catering business that calculates the number of meatballs he needs to prepare. the formula is m=4a+2c, where c=number of children; m=number of meatballs; a=number of adults. how many meatballs are required for a party of 10 adults and 5 children?

Will mark brainliest! ( explain the answer too, it can be one sentence)