Red = r
Blue = b
Green = g
Step 1: Write as algebraic equations.
r = b + 6
g = b - 4
Step 2: Merge the two equations together to form one, by adding them together.
r + g = b + b + 6 - 4
Step 3: Simplify.
r + g = 2b + 2
Step 4: Make b the subject.
2b + 2 = r + g
2b = r + g - 2
b = 0.5(r + g - 2)
ANSWER
b = 0.5(r + g - 2)