Question

D= c-s/n solve for n

Answer 1

Step 1: d = c-s/n
Step 2: dn = c-s
Solution is n = c-s/d :-)

Answer 2

D=c-s/n
Subtract c too both sides
d-c=-s/n
Multiply -s too both sides
d-c*-s=n
Final answer
n=d-c*-s