Two parallel lines do not cross and therefore have the same slope. For the line y=2x+1, the slope is 2. Substitute 2 and the point (2,-1) into the formula
y = 2x - 5
The equation has to have the same slope -- in this case, it is 2 -- to be parallel to 2x - y = 4.
Now we have y = 2x - k.
Then, we plug in the coordinates to our new equation to solve for k.
-1 = 2(2)-k
-1 = 4-k
k = 5
So the final equation would be y = 2x - 5