a) $\sqrt{9}$ means “the square root of 9”, or in other words, “the number that, when multiplied by itself makes 9”. So, the answer is 3.
b) $\sqrt[3]{216}$ means “the cube root of 216”, or in other words, “the number that when cubed gives you 216”. You are expected to recognise the first few cube numbers, but if not, it’s worth just trying to cube some smaller numbers until you find one that works.
c) $\sqrt{25}$ means “the square root of 25”, or in other words, “the number that, when multiplied by itself makes 25”. So, the answer is 5.