You can use the power rule to differentiate the expression:

The derivatives of and are:

And since they represent the instantaneous velocity of the particle, you are looking for when

Solving for t, I get complex answers... so that would mean the particles never travel at the same speed.

In fact, knowing that the gradient of a function is its rate of change, you can graph and observe the original two functions and and notice that they are cubics which are a reflection of one another. In , the gradient isalwayspositive. In , the gradient isalwaysnegative. This shows that their gradients and hence their rates of change can never be equal.