n=i*x+y for i
This is a literal equation.
Subtract y from each side of the equation:
n - y = i*x + y - y
The y's cancel on the right side, so we have:
n - y = ix
Divide each side of the equation by x, to isolate i
(n - y)/x = ix/x
The x's cancel on the right side, so we have:
i = (n - y)/x