Question

a + s = d, solve for s
d + a = s
da = s
a - d = s
d - a = s

Explanation:

Step1: Isolate s

Subtract a from both sides of the equation $a + s=d$.
$a + s - a=d - a$

Step2: Simplify

$s=d - a$

Answer:

$d - a=s$