Four ratios:
light to dark: 4:12
dark to light: 12:4
black to white: 2:4
white to gray: 4:10
gray to white: 10:4
any of those work.
Solve for the first variable in one of the equations, then substitute the result into the other equation.
Y
=
2
y
/3
−
3
x
=
−
y
/3
+
7
Hope this helped!
The answer to that is y = -6