We will be using #e^x= sum_{n=0}^{infty}x^n/(n!)#
Taking #e^{3t} = 1 + 3t+(3t)^2/2+(3t)^3/6+...# and substituting #lim_{t->0}(e^{3t}-1)/t = lim_{t->0}((1+ 3t+(3t)^2/2+(3t)^3/6+...-1)/t)# #lim_{t->0}(e^{3t}-1)/t =lim_{t->0}(t((3+(3t)/2+(3t)^2/6+...))/t)=3#