Hello all, i'm new to differentiation and i'm totally confused.

1.) A space vehicle moves so that the distance travelled over its first minutes of motion is given by y = 4t^4, where y is the distance travelled in metres and t the time in seconds. By finding the gradient of the chord between points where t = 4 and t = 5, estimate the speed of the space vehicle when t = 5.

2.) Could someone help me define the "limits" and "derivatives" in this topic

Thanks.