substitute x in the equations for numbers of hours worked, and you should look for the one that gets the right amount of money.
If you use 120 hours, you should look for the equation that gives you the answer of 2700:
f(x)=120x+400
120(120) + 400 =14800
f(x)=400x
400(120) = 48000
f(x)=100x+300
100(120) + 300 =12300
f(x)=20x+300
20(120) + 300 = 2700
Just to check, use the other number of hours: 100 hours.. You should expect to see an answer of 2300:
f(x)=20x+300
20(100) + 300 = 2300
the equation you want is f(x)=20x+300