Matt is right, but there is another way to do it as well.
if you do not want to distribute the s then you can do it this way:
A=s(1-DN)
divide both sides by s, A/s=1-DN
subtract the one, (A/s)-1=-DN
divide by -D, [(A/s)-1]/-D=N
or if my format was too confusing, (A/s)-1 ÷ -D=N
