Originally Posted by

**startingover** OK I am getting really close now...the book shows a negative answer, which I am unsure of.

The velocity v(t) = x'(t) at time t of an object moving along the x axis is given, along with the initial position x(0) of the object.

Find:

the position x(t) at time t

the position of the object at time t=4

the time when the object is at x=3

x'(t) = -2(3t+1)^1/2, x(0) = 4

y=u^1/2

dy = 1/2u^3/2du

x=3t+1

dx = 3dt

x(t) = 1/2 u^3/2 (3t+1)^3/2 dx

x(t) = 4/9(3t+1)^3/2 + C

book says 4/9 should be negative.

Unsure of how to do the position of the object at t=4 and time of x=3. The way I have tried to substitute isn't coming up with the books answer.