Electrons movement?

Question

An electron's position is given by r = (6.0t - 4.0t^2) i+ 2.0 j with t in seconds and r in meters.

When does the speed equal 10 m/s?

I've been trying for 30min to find it but I can't... :(

Answer 1

Take the derivative, set equal to ten. Solve and you get t=-1/2 sec.

Answer 2

v = dr/dt =( 6 -8t)i

when v =10m/s

( 6 -8t)= 10

-8t = 4

t =(-1/2) s