Find Curvature of Ellipse given x=3*cos(t) and y=4*sin(t) at the points (3,0) and (0,4)

Relevant equations: curvature at r(s) is k(s)=||dT/ds|| when r(s) is arc length parametrization and T is the unit tangent vector
I usually use the formula k(t)= (||r'(t) x r''(t)||)/||r'(t)||^3

So, r(t)=<3cost,4sint> and r'(t)= <-3sint,4cost> and r''(t)=<-3cost,-4sint>
do I just plug them in for k(t)? I think I'm missing a step, someone help please. Thank you