Okay, so write it as 2x - 10y - 20 = 0
divide both sides by 2:
x - 5y - 10 = 0
We can ignore the -10 because that is not affecting the gradient of the line, just where it crosses the y axis. This leaves us with x-5y=0 or x = 5y this means that as x increases by 1, y increases by 5, so the gradient is 5.