dx/dt=2t,dy/dt=3t^2,
dy/dx=(dy/dt)/(dx/dt)=3t^2/(2t)=3t/2,
t=2时,直线斜率为:dy/dx=3,
y=8,x=4,
(y-8)/(x-4)=3,
切线方程为:y=3x-4.