Change the subject of the formula s=n/2 (a+l)

If you want n to be the subject of the formula;

n=2s (a+l) = 2sa+2sl

if you want a to be the subject of the formula;

n=2sa+2sl

2sa=n-2sl

a = (n-2sl) / (2s)

if you want l to be the subject of the formula

n=2sa+2sl

2sl=n-2sa

l = (n-2sa) / (2s)