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 is always positive. In , the gradient is always negative. This shows that their gradients and hence their rates of change can never be equal.