For these problems you have to use PEMDAS so for problem 15 you would do the parentheses first, (21-3) you get 18 and then exponents, 3^2, you get 9, no multiplication but there is division so take your answers, 18 and 9 and divide them, you get 2, so the answer to 15 is 2.
Circumference of a circle = 2πr
So,

Answer: 
<em>Your Answer Is...</em><em> </em>2
Step-by-step explanation:
<em>Simplify the radical by breaking the radicand up into a product of known factors, assuming positive real numbers.</em>
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Hopefully this helps <em>you!</em>
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<u>- Matthew ~</u>
Like terms must have the same variables and those variables must have the same powers.
and
are like terms, because they both have
and they both have
.
and
are NOT like terms, because the powers no longer match.
In your example, do you have the same variables and do the variables have the same powers in both expressions?
Answer: 0.2 fam §(* ̄▽ ̄*)§