If its average speed is .79 miles in an hour, then it goes .79 miles in an hour.
If you add them straight down:
2d-e = 7
d + e = 5
Then the e's cancel out.
You are left with 3d = 12
Then solve for d by dividing each side by 3.
Now you have d =4.
To solve for e go back into one of your original equations and substitute 4 for the d.
You can use either equation, so I'll choose the second one.
4 + e = 5
Now solve for e by subtracting each side by 4.
Now you have e = 1.
So your solutions are d = 4 and e = 1
Perimeter is the sum of all the sides of the polygon :)
