Question 909081
You made an error. When you went from {{{16(3a+m)^2 - (5x-10y)^2}}} to {{{(4(3a+m)+5x-10y)(4(3a+m)-5x-10y)}}} you made a mistake.


It *should* be this


{{{16(3a+m)^2 - (5x-10y)^2}}}


{{{(4(3a+m) + (5x-10y))(4(3a+m) - (5x-10y))}}}


{{{(4(3a+m) + 5x-10y)(4(3a+m) - 5x+10y)}}}


notice on the last line I have -5x+10y (whereas you have -5x-10y). So it was a case of you not distributing the negative through fully.


I'll let you finish up.