An ion beam has a cross-sectional of 2.5 mm^2 and the charge delivered by it to a target can be expressed as q(t)=(.12 C/s^3)t^3 + (.24 C/s^2)t^2 - .45 C. Calculate the current density in the beam at t = 3.0 seconds.

Can anyone help me with this? I know I have to do an integral from 0 to 3, and plug it in to I formula to get the I and then plug that into current density formula j = I/A.
Thanks.