[tex] b\ -\ black\\r\ -\ red\\\\\dfrac{b}{r}=\dfrac{6}{4}\leftarrow\text{the constant of proportionality}\to \text{in simplest form}\ \dfrac{6:2}{4:2}=\dfrac{3}{2}\\\\\dfrac{b}{r}=\dfrac{3}{2}\ \ \ \ |\cdot r\neq0\\\\b=\dfrac{3}{2}r\leftarrow\text{the equation} [/tex]