$$\dfrac {3x+2y}{4z}+\dfrac {5x-6y}{3z}$$

After taking LCM, this becomes

$$\dfrac {3z\left( 3x+2y\right) +4z\left( 5x-6y\right) }{12z}$$

By cross multiplication, we get the same thing. However, this works only when we have two fractions to add or subtract. LCM works for any nymber of fractions.