how to solve a=7a+(n-3)d for d

a = 7a + (n - 3)d Given

a - 7a = 7a + (n - 3)d - 7a Subtract 7a from each side

-6a = (n - 3)d Simplify each side

-6a/(n -3) = (n - 3)d/(n -3) Divide each side by n - 3

-6a/(n - 3) = d Simplify the right side

d = -6a/(n - 3) Symmetric Property

d = -6a/(n - 3) * (-1/-1) Multiply the right side by -1/-1

d = 6a/(-n + 3) Simplify

d = 6a/(3 - n) Commutative property of addition