Step-by-step explanation:
the function actually is
y = 40(x - 1) + 70 = 40x - 40 + 70 = 40x + 30
still, the slope (the factor of x) is 40.
that means y (the cost) increases by 40 for every additional hour (x increases by 1).
so, the rate of change (slope) is 40/1 = $40 per hour.