# Centre Of Mass: Frame

I don't understand how you would find the centre of mass of $CD$ from $AB$? Thanks in advance.
For the distance from CD, you need only the equation of CD which is simple enough (y-0) = (4/3)(x-4) or 4x - 3y - 16 = 0. This leads directly to the solution $\frac{|4(a)-3(b)-16|}{5}$.