Solve for a S=a+(n-1)d

$S = a + (n - 1) d$

Rewrite the equation as $a + (n - 1) d = S$ .

$a + (n - 1) d = S$

Simplify each term.

Apply the distributive property.

$a + n d - 1 d = S$

Rewrite $- 1 d$ as $- d$ .

$a + n d - d = S$

Move all terms not containing $a$ to the right side of the equation.

Subtract $n d$ from both sides of the equation.

$a - d = S - n d$

Add $d$ to both sides of the equation.

$a = S - n d + d$