81x^2 - 25 = 0

9x^2 - x = 0

4x^2 + 5x - 6 = 0

1) 81x²-25=0

(9x) ²-5²=0

(9x+5) (9x-5) = 0

a) 9x+5=0, x=-5/9

b) 9x-5=0, x=5/9

2) 9x²-x=0

x (9x-1) = 0

a) x=0

b) 9x-1=0, x=1/9

3) 4x²+5x-6=0, D=25+96=121, √D=√121=11

x1 = (-5+11) / 8=6/8=3/4=0,75

x2 = (-5-11) / 8=-16/8=-2