You problem is you don't have any problem. You have a bunch of formulas for the perimeter and area of some shapes.
As for using the formulas, usually you're given all but one of the variables and you can solve for the remaining one.
Notice how the triangle result doesn't have area as a function of a, b and c. That's called Heron's formula and is usually not taught to secondary school students. I don't like to teach it either because Archimedes' Theorem is so much better:
The sides a,b,c and area S of a triangle satisfy

He payed $16 for the files because 48-13-19= 16 You have to subtract the total cost to the prices you have been given for each item to find the missing value! :) Hope this helps
The sum us 15. Hope this helps.
$44.52 is the final cost of a flower bouquet.
Answer:
INCORRECT
Step-by-step explanation:
Given a rectangular prism,
Total Surface Area = 2(LW+LH+WH)
If Length = 12, Width =20, and; Height = 16.
Total Surface Area = 2(12*20+12*16+20*16)
=2(240+192+320)
=2(752)
=1504
The Surface Area is 1504 Square Units, therefore if someone says the surface Area is 3,840, they are incorrect!