15a-25b = 5*(3a-5b)
3a²+ab = a*(3a+b)
28c+21b = 7*(4c+3b)
4dc²-2c = 2c*(2dc-1)
x^4-8x^2-9=0,
x^2=a,
a^2-8a-9=0,
По теореме Виета,
a1=-1<0, a2=9,
x^2=9,
x1=-3,
x2=3.