If you are student of some technical school then shame on you. If you are part of some humanistic studies then you should not slept at math classes few years back.
add line:
my_var =sqr(k/m)
then your formula is:
v = A * my_var * cos (my_var *t)
and check if m != 0 else you can divide by zero and it will be end of our world!
ps.
Your severe lack of logic (as for uni student) is disturbing.