Since there are two unknowns to solve, you will need a minimum of two equations.

First equation makes use of the info that the speed is 48cm/s when t =2. As speed is obtained by differentiating distance with respect to time, this gives you the first equation.

Second information makes use of the info that the speed is maximum when t = 2. Stationary points, such as maximum points, can be found by equating the differential to 0. By differentiating the speed equation or finding the second derivative of the distance equation (same btw), you will be able to form your second equation.

A good practice which I didn’t include would be to confirm that the point is indeed a maximum point.