<span> (4х+5)(3-х)=х^2 - 2х</span>
<span>23x - 4x^2 +15 - 5x - x^2 +2x = 0</span>
<span>-5x^2 +20x+15 = 0</span>
<span>-x^2 + 4x +3=0</span>
<span>a = -1</span>
<span>b = 4</span>
c = 3
5(x-3)(x+7)=5(x^2-3x+7x-21)=5(x^2+4x-21)=5x^2+20x-105
An= 4n-17, a(n+1) = 4(n+1) -17=4n+4-17=4n-13
d= a(n+1)-an = 4n-13 - 4n+17 = 4,d=4