Solve the equation a=m-n for the variable n.

a=m-n

Add m

a+m=-n

Divide by - 1

a+m/-1=n

Our objective is to isolate n one side of the equation. The steps are as follows ...

a=m-n

(Add n to both sides)

a+n=m-n+n

(The - n and + n cancel out, on the right side of the equation)

a+n=m

(Now subtract a from both sides)

a+n-a=m+a

(The + a and - a cancel out, on the right side of the equation)

n=m+a