This is a linear equation.
y = mx+b
y is the total cost.
The slope m is the rate: that's 9 dollars per hour.
x is the number of hours.
b is the initial value: that's 16 dollars.
y = 9x+16
The problem states that the cost was 79 dollars.
Substitute that for y and solve for x.
79 = 9x+16
63 = 9x
x = 7 hours
