this is how i solve can u point out at what am i doing wrong

v=V-u equation 2

mv+MV=mu equation 1

multiplying eq 2 by M

we get Mv=MV+Mu equation 3

adding equation 3 and 1 we get

v=(m-M)u/(m+M) equation 4

multiplying eq 2 by m

mv=mV-mu equation 5

adding equation 5 and 1 we get

V=2mu/(m+M)

plugging these values in the energy loss in kinectic energy

mu^2{1-(m+M)2-4mM/(m+M)^2 i get 0/(m+M)^2 i know i am close but what i am doing wrong??