Subtract 2x from each side:

2x - 2x - b/y = 4c - 2x

Cancel the 2x's on the left side and we get:

-b/y = 4c - 2x

Cross multiply:

-b = y(4c - 2x)

Divide each side by (4c - 2x):

-b/(4c - 2x) = y(4c - 2x)/(4c - 2x)

Cancel the (4c - 2x) on the right side and we get:

**y = -b/(4c - 2x)**