Solve the system ... y=3x-8 and y=-9x+4

1] y - 3x = -8
[2] y + 9x = 4

-3x + y = -8 9x + y = 4

Solve equation [2] for the variable y

[2] y = -9x + 4

// Plug this in for variable y in equation [1]

[1] (-9x+4) - 3x = -8
[1] - 12x = -12
// Solve equation [1] for the variable x

[1] 12x = 12

[1] x = 1
// By now we know this much :

y = -9x+4
x = 1
// Use the x value to solve for y

y = -9(1)+4 = -5

{y,x} = {-5,1}

The answer would be 13.5 minutes.