n = b + d^2a for a

n = b + d^2a for a

Let's start by isolating the one term with the a variable.

Subtract b from each side:
n - b = b - b + d^2a

Cancel the b terms on the right side and we get:
n - b = d^2a

With the a term isolated, let's divide each side of the equation by d^2:
(n - b)/d^2 = d^2a/d^2

Cancel the d^2 on the right side, and we'll display this with the variable to solve on the left side:
a = (n - b)/d^2