y = (1000*0.96^t)* e^3.247 * e^(0.0873*80(1-0.96^t))
Any help muchly appreciated. Thanks

y = k.a^t.e^(bt) for some a,b,k (using . for multiplication)

note that d/dt(a^t) = d/dt(e^t.ln(a)) = ln(a).e^t.ln(a) = a^t.ln(a)
and therefore by the chain rule
d/dt(e^(bt)) = b^t.e^(bt).ln(b)

so using the product rule:

d/dt(y) = k.{ a^t.ln(a).e^(bt) + a^t.b^t.ln(b).e^(bt) }

now you need to evaluate a,b,k from your original expression and plug them in.