[tex]21 {y}^{7} \div {( - 3y)}^{4} = 21 {y}^{7} \div [ {( - 3)}^{4} \times {y}^{4} ][/tex]
[tex] = 21 {y}^{7} \div 81 {y}^{4} = (21 \div 81) \times ( {y}^{7} \div {y}^{4} )[/tex]
[tex] = \frac{21}{81} \times {y}^{7 - 4} = \frac{7}{27} \times {y}^{3} = \frac{7 {y}^{3} }{27} [/tex]
