Velocity = rate of change in displacement/change in time
v = dx/dt which is the derivative of 'x' with respect to 't'
= d(7.8)/dt + d(9.2t)/dt - d(2.1t^3)/dt
since d(a+b+c)/dt = ...
This solution provides step by step calculations for finding the velocity based on the derivative of 'x' with respect to 't'.