Answer:
If it is blue marbles than probably 30. White marbles, then maybe 20.
Step-by-step explanation:
W = L - 7
WL = 78
(L-7)L = 78
L^2 - 7L - 78 = 0
Factorize: (L-13)(L+6) = 0
L = 13 or -6
A length cannot be negative, so the length must be 13 miles.
The width is 13-7 = 6 miles.
Answer:

Step-by-step explanation:
The y-intercept is where the line itersects the y-axis, which is the dark vertical line. This is -8 represented as the constant in the equation.
To find the slope, find the rise over run. Every time the line goes over 1 it goes down 6, so the slope would be -6.
We have:
5(x + 7) = 85 |divide both sides by 5
x + 7 = 17 |subtract 7 from both sides
x = 10