4.

а)

2*(3х-2у)(3х+2у) = 2*(9х²-4у²) = 18х² - 8у²,

б)

(4у²+9)(2у-3)(2у+3) = (4у²+9)(4у²-9) = 16у⁴ - 81,

в)

(7m²-3n³)(7m²+3n³) = 49m⁴ - 9n⁶,

5.

а)

25у² - 16 = 0,

25у² = 16,

у² = 16/25,

у = ±√(16/25),

у1 = 4/5,  у2 = -4/5,     (или  у1 = 0,8,  у2 = -0,8),

б)

(х - 2)(х + 2) - х(х + 5) = -8,

х² - 4 - х² - 5х = -8,

5х = 8 - 4,

5х = 4,

х = 4/5   (или  х = 0,8)