Consider what is done to u.
- z is subtracted
- the difference is divided by 2
When you solve for u, you "undo" these operations in reverse order.
Division by 2 is undone by multiplying by 2. You do this to both sides of the equation.
... 2×F = 2×(u-w)/2
... 2F = u - w . . . . . . . simplified
Now, w is subtracted. You undo that by adding w to both sides of the equation.
... 2F + w = u - w + w
... 2F + w = u . . . . . . .simplified
If you like, you can swap sides so that u is on the left side of the equal sign.
... u = 2F - w
