Charge(x) = Electrician's charge as a function of the # of hours worked.
Here,
Charge(x) = $50 + ($40/hour)x
Set this expression = to $200 and solve for x, the number of hours for which the electrician must be paid:
$50 + ($40/hour)x = $200
To solve for x, subtract $50 from both sides: ($40/hr)x = $150.
Then x = 3.75.
You cannot afford to have this 4-hour job done, because your $200 will cover only the house call and the first 3.75 hours.