See picture for solution to your problem.
20 cm is the answer good luck
To do this, we're going to use the order of operations (PEMDAS):
P - Parentheses
E - Exponents
M - Multiplication
D - Division
A - Addition
S - Subtraction
First let's do parentheses, there isn't anythig in parentheses we need to simplify, so we can skip this step.
Next let's look for exponents. I see we have a
so let's replace that with
:

Now let's look for multiplcation. We know that things that are right next to eachother in parentheses represent multiplcation, so let's simply this more:



And now we're left with a simple problem we know how to solve.
Answer: 
Hope this helps!
I wrote ones but I hope it’s right and helps