Infinity isn't actually a number - it's a concept that represents that the set of numbers goes on forever. As a result, you can't take the square root of infinity. However, you could state that as x approaches infinity (or rather, increase indefinitely), the square root of x also approaches infinity (again, increases indefinitely).
It's a proportion. 2 is to x as 15 is to 9, or 2/x = 15/9. Multiply the means and extremes 15x = 18 divide both sides by 15 to get x by itself. x = 1.2
