lg²x -5lgx +6 = 0
lgx = t => t²-5t+6 = 0 => t₁ = [5 + √(25-24)]/2 = (5+1)/2 = 3
t₂ = (5-1)/2 = 2
lgx = 3 <=> lgx = lg10³ => x = 1000
lgx = 2 <=> lgx = lg10² => x = 100
