Name the like terms in the expression.
16x + 12y^2 + 4a + 12x

The 'like' terms are the  16x  and the  12x.
They can be combined to make a single term ...  28x .

The like terms are 16x and 12x 

C= 2*pi*r
pi= approx. 3.14
So
The first one with the radius being 18 would be approximately 113.04
BECAUSE 18 * 2 = 36
36*3.14= 113.04
DO THE SAME PROCESS with the other values you were given...

Also if they give you the diameter the formula is C= d*pi
that formula is the same as c=2*pi*r  Use either... just remember when they give you the radius the radius is half of the diameter... so take the radius times 2 and times that by pi 
When it asked for an approximation use 3.14 for pi.
I'm sure you can do the rest by yourself. I hope this helped!