The graph shows the relationship between the distance a truck can travel and the amount of gasoline used.

what is the unit rate for the situation?

a.
mpg

b.
7 mpg

c.
35 mpg

d.
175 mpg

Slope - Intercept form formula: y = mx + b
'm' represents the slope (unit rate)
'b' represents the y - intercept

Equation: y = 7x
Slope (unit rate): 7
Y-Intercept: 0

If you used 5 gallons of gasoline, you go 5 times the number of miles, which is 7, to get a total amount of 35 miles.
Unit rate = 7
The answer is B.

The graph shows the relationship between the distance a truck can travel and the amount of gasoline used.

Unit rate is define as the rise over the run or distance covered per unit time.
In this case : 
Rate = 28 / 4 = 7 mpg