Take the derivative twice then substitute t=3 into the equation
x' = -12sin(5t) + 4cos(5t)
x" = -12cos(5t) - 4sin(5t)
= -12cos(15) -4sin(15)
= -12.62 units/time^2 I think.
Guest is right....Thanx for the correction !
x' = - 60 sin (5t) + 20 cos(5t)
x" = - 300 cos (5t) - 100sin (5t)
when t = 3 then x" = -315.66