The domain is the input values, which are the x values.
The x values in the given pairs are: 8, 0,4,-3
The domain set is (-3, 0, 4, 8)
L (water level) = -0.5 d (days) +34
Place 26 into the (L) variable and solve
26= -0.5 d + 34
0.5 d = 8
d=16
I rather not..............
