Peter G. answered 10/10/16
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It looks like you left out b in the numerator when doing the quotient rule in the first part.
For the second portion, notice you have ef(t), where f(t) = -2e2t. Then note f'(t) = -4e2t, and
d/dt 6*e^(-2e^2t) = 6 d/dt ef(t)
= 6 f'(t)ef(t)
= -24e2te^(-2e^2t).