You did that correctly. To find your 2nd derivative just take the derivative of your first derivative.
the problem is:
if v(t)=sqr root(t^7)-sqr root(t) find the second derivative v"t.
i differentiated it and came up with 7/2t^5/2 - 1/2t^-1/2. but what does it mean to find the second derivative? and am i even doing it right?