Lines get stiff and shrink over time from exposure to gas. Therefore to stop this type of leak, it's a good idea to replace both lines. If you're doing this repair yourself, drain the tank first, then push the lines off the carburetor connections using the side of the blade of a screwdriver. Next, pull them through the grommet and remove them from the tank. Remove the grommet also, ideally using your thumb to peel it out by the edges rather than using a screwdriver which could mark the sealing surfaces, potentially resulting in a leak when it's replaced. Bring the trimmer or grommet to your local hire/repair shop and get them to provide you with a fuel line that's a tight fit for the grommet. When replacing the lines, feed them through the hole in the tank first with the grommet removed. Then push both lines through the grommet, leaving about 1/2 inch feeding through. It's easier to pull lines than push because they stretch and reduce slightly in diameter, whereas they bulge when being pushed. Next replace the grommet in the tank. If you find this difficult, use a screwdriver to poke it in at the edges, but use the side of the screwdriver blade rather than the tip, again to avoid damage. Pull out the lines and connect them to the carburetor inputs, making sure they're the correct way around.