4.
[tex]a)9c {}^{2} - 49d {}^{2} = 3 {}^{2}c {}^{2} - {7}^{2} {d}^{2} = (3c) {}^{2} - (7d) {}^{2} = (3c - 7d) \times (3c + 7d) [/tex]
[tex]b)9x - 54x {}^{4} = 9x \times (1 - 6 {}^{3} )[/tex]
