The calculation is:
I see you have doubts, so I will explain each step.
Step 1: simpliy 576 / 64. In this case when you divide 576 by 64 you get 9 that is a whole number, so that is its simplest form.
Step 2: Calculate the square root of 9, which is 3.
If you use a simple calculator you will obtain that, but the square root of 9 is a calculation that you should know by yourself.
Remember that the square root a number N is the number that multiplied by itself results in the number N. For example the squere root of 4 is 2, because 2 times 2 is 4. So, the square root of 9 is 3 because 3 times 3 is 9.
It will be very helpful for you to learn the 12 first squares because with that you will be able to tell the square roots of the most elemental numbers:
1^2 = 1 => √1 = 1
2^2 = 4 => √4 = 2
3^2 = 9 => √9 = 3
4^2 = 16 => √16 = 4
5^2 = 25 => 25 = 5
6^2 = 36 => √36 = 6
7^2 = 49 => √49 = 7
8^2 = 64 => √64 = 8
9^2 = 81 => √81 = 9
10^2 = 100 => √100 = 10
11^2 = 121 => √121 = 11
12^2 = 144 => √144 = 12