State what you know and what you don't know:
Let x = the number of tires installed
Let 40 = total number of installations
Let 40 - x = the number of gears he assembles
Let 2x = earnings on tire assemblies
Let 5(40 - x) = earnings on gear assemblies
Let 110 = total earnings
Use the last three statements to set up your equation.
2x + 5(40 - x) = 110
I think you might be able to take it from here.