I just cant figure out this question and the paper is due soon! i need help asap can some one please help me?
here is the Irrigation question:

What amount of water can be used by a plant prior to scheduling the next irrigation in a turfgrass yard growing on sandy soil with a 4 inch root zone?

Step-by-step explanations

Clay 1.5 0.12 30

Silty Clay 1.9 0.18 40

Clay Loam 2.4 0.25 40

Silty Clay Loam 2.4 0.25 50

Sandy Clay Loam 1.8 0.20 50

Sandy Clay 1.9 0.12 50

Silt 2.0 0.40 50

Silty Loam 2.4 0.43 50

Loam 2.0 0.54 50

Sandy Loam 1.4 0.75 50

Loamy Sand 0.8 0.88 50

Fine Sand 0.7 1.25 60

x = -3

y = -18

step-by-step explanation:

to solve by substitution, substitute one variable and solve for the other. then substitute again to find the remaining variable.

x-1/3(3x-9) = -3 since y=3x-9 substittute y.

x-1/3(3x-9)=-3

x + x +3 = -3

2x+3=-3

2x+3-3= -3-3

2x=-6

x=-3

substitute again into y=3x-9

y=3(-3)-9

y=-9-9

y=-18