Presumably you mean the second derivative of y with respect to x, d²y/dx².
Compute the first derivative. By the chain rule,
Differentiate the two parametric equations with respect to t :
Then the first derivative is
Now, dy/dx is a function of t, so we can denote it by, say, dy/dx = f(t). Then by the chain rule, the second derivative will be
Differentiating f(t) :
Then the second derivative is
and since y = cos(t), we can go on to say