A couple of things you'll need to clarify.
Is your cos treating the number in parentheses as being in radians or
degrees? Excel defaults to radians.
You may need further parentheses, for example for which term you are going
to take the square root. With some dimensional analysis I guess that pehaps
the sqrt is of (B^2-a^2) ?
If your four constants are in C$1 to C$4, and your values of t from A2
downwards, your values of y in B2 downwards could be
=C$1*EXP(-C$2*A2*COS(A2*SQRT(C$3^2-C$2^2)+A2*C$4))
