The standard math library cmath includes a function for taking the square root of a number. The heading (prototype) for this function is:
double sqrt(double x)
Write another function called closer_root that takes two real numbers and returns the one whose square root is closer to the number 10. Think about how you would do the problem first, it's simple math, but easy to get confused if you don't pay attention.
You also might find the standard absolute value function useful:
double abs(double x)