Answer: The mechanic worked for 4.25 hours for the fourth customer.
First find out the slope.
Y^2-y^1/x^2-x^1
210-90/3-1=60
Slope: 60
Then find out what the y-intercept is by plugging in values we know into y=mx+b
90=60(1)+b
30=b
Y=60x+30
Substitute the y with 285 to find out the x (hours)
285=60x+30
4.25=x