Solve u^2=-9 where u is a real number.

Taking the square root of both sides, we have:

u = +/- √(-9)

We can simplify this expression by breaking down the square root of -9 into the square root of -1 times the square root of 9:

u = +/- √(-1) * √(9)

Since the square root of -1 is denoted as "i", and the square root of 9 is 3, we can simplify further:

u = +/- 3i

Therefore, the solution to the equation u^2 = -9 where u is a real number is u = 3i or u = -3i.