The equation for area of a circle is (pi)(r^2) and the circumference of a circle is equal to (2)(pi)(r).
Pi is commonly equal to about 3.14, but if you plug it into a calculator it is much more precise.
R is the radius of a circle, or half of the length across (aka diameter) of a circle.
So r = 3 or 1/2 of the diameter of 6.
If you plug 3 into both equations you get (pi)(9) =28.26 (roughly) and 2(3)(pi) = 18.84 (roughly).
The equation says these two values must be added (a+b) so, 28.26 + 18.84 = 47.1
Note: if the answer has the pi symbol in it, it just means you add 9(pi) + 6(pi) without changing the pi symbol to 3.14, which is equal to: 15(pi)
Hope this helps! :)