Substituting a value for a differential function



Hi,

I have a function below.  I am trying to subsitute t=25 in the dC(t) and I
am getting an error. Please let me know what I missing ot get the value of
the differential function.

C(t):=(Ax^2)+(Bx)+C
dC(t):=(diff(C(t),t,1))$

Thanks your time and help.
Vishraj