Помогите решить задачу по алгебре

(x² + 1)/5 – (x + 1)/4 = 1
4(x² + 1) – 5(x + 1) = 20
4x² + 4 – 5x – 5 = 20
4x² – 21 – 5x = 0
4x² + 7x – 12x – 21 = 0
x × (4x + 7) – 3(4x + 7) = 0
(4x + 7) × (x – 3) = 0

4x + 7 = 0
x – 3 = 0

x1 = –1,75,
x2 = 3.

a
x(x-9)(x+9)=0
x1=0
x2=-9
x3=9