[tex](5 + 4x)^2 = (9 - 21x)(4x + 5)

25 + 40х + 16х^2 = 36x - 84x^2 + 45 - 105x

40x + 16х^2 - 36x + 105x + 84x^2 - 45 + 25 =0

109x + 100x^2 - 20 = 0

100x^2 + 109x - 20 = 0

D = b^2 - 4ac

D = 11 881 + 8000 = 19 881

x= (-b ± √D)/2a x= (- 109 ±141)/200

x1 =(- 109+141 )/200= 32/200=16/100=0.16

х2 = (- 109-141)/200= (- 250)/200= - 125/100= -1.25