The formula is 1/2 x P x L + B
P=perimeter
L=Side length
B= Slanted Base length
ANSWER= 8 mi
Suzy: (7 days)(5/4 miles per day) = 35/4 miles
Gretchen: (7 days)(3 miles per day) = 21 miles
Subtracting, 21 miles less 35/4 miles = 49 miles (difference)
x => Hamburgers
y => Hot dogs
x + y = 26
3.5x + 2y = 70
-2x - 2y = -52
3.5x + 2y = 70
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1.5x = 18
x = 18 / 1.5 = 12
y = 26 - 12 = 14
They sold 12 hamburgers and 14 hot dogs
we can mark points on the graph shown , check the image I have shared with this for the points .
Formula for slope = 
So for Line A points are (1,3) and (0,-1)
So using t he formula slope is = 
slope of line A =4
For Line B the points are (0,4) and (3,-5)
So using t he formula slope is = 
slope of line B = -3