Answer:
7x(y - 7)(y + 7)
Step-by-step explanation:
[tex]7x {y}^{2} - 343x \\ \\ = 7x( {y}^{2} - 49) \\ \\ = 7x ( {y}^{2} - {7}^{2} ) \\ \\ = 7x(y - 7)(y + 7)[/tex]
