I know this has been answered but here is an alternative method.
we only require the first two terms of the expansion to find the coefficient of x^5. why ?
in the first term you can use the binomial formula for the coefficient of the x^5 which is 56
for the next term it should be obvious that only three terms form both brackets multiply to give a term of order 5. they are x^5 with 1 , x^3 with 6x^2 and 3x with x^4.
P.S who killed latex ?
PPS who killed the image tags ?