To express $2:3$ and $4:3$ in the form $a:b:c$, the subsequent of the first is made equal to the antecedent of the second using their L.C.M. equal to—

The subsequent (consequent) of the first ratio is $3$ and the antecedent of the second is $4$; to equalize them we use their L.C.M., which is $\text{LCM}(3,4)=12$.