I've been given the following equation to graph, plus a couple of sub-problems I need to solve, and I'm having trouble solving one of them. A quick summary of the problem:

A speeding projectile observed a time 0 can be represented by the graph 
$y = x^2 + 2$
for the range of -2 seconds to 4 seconds.
a) plot the graph (already done)
b) how long does it take to reach 15 m/s

It's b) that I'm having the problem with. I thought this would be as simple as taking the equation and turning it into

$15 = x^2 + 2
13 = x^2
\sqrt(13) = 3.61
x = 3.61$

But I'm not sure if I'm doing this right. Am I missing something here, or is this correct?
$X$ is supposed to be time, $Y$ is speed.