That's the wrong answer.

If you mean this:

a-4b(a+b)

then we distribute only the -4b:

a-4ba-4b²   

and since "a" comes before "b" in the alphabet

we write "ba" as "ab" to keep alphabetical order:

a-4ab-4b²   <--final answer

---------------------------

However if you meant this

(a-4b)(a+b)


then we must first write the second parenthese
beside each of the two terms in the first 
parentheses like this:  

a(a+b)-4b(a+b)

Then we distribute twice, once with the "a"
over (a+b), and then again with the -4b over
the (a+b)

a²+ab-4ba-4b²

Now since "ba" is the same as "ab" we write

a²+ab-4ab-4b²

Finally since +ab and -4ab are like terms,
we combine the coefficients and get

a²-3ab-4b²   <--final answer

Edwin