I have El capitan but safari stopped opening tells me can't find application something wrong with extension

Can you open it by accessing it in the Applications folder? If so, you may have a corrupt Dock alias. Click and hold the alias and drag it about 1 inch away from the Dock and then release. Then drag the application from the Applications folder to the Dock to create a new alias. When you have a new alias, control - click/Options/Keep in Dock.