Given

Mx(t) = exp(t^2 + 2t)

My(t) = exp(3t^2 + t)

Find the mgf of X + 2Y.

___________________________

Can anybody explain how

My(2t) = exp(12t^2 + 2t) ???? I figured this going backwards from the answer...so it may be wrong...

THANKS

____________

The answer is exp(13t^2 + 4t)