a) 25-5x+3-4=0
=> 24-5x=0
=> 5x=24
=> x=\(\dfrac{24}{5}\)
b) 259-51/4x+3/=100.3+10=310
=> -51/4x+3/=310-259=51
=> /4x+3/=-51 (1)
Vi /4x+3/≥0 (2)
Tu (1)(2) => pt vo nghiem
c) 7x-21-15+5x=6
=> 12x-36-6=0
=> 12x-42=0
=> 12x=42
=> x=\(\dfrac{42}{12}=\dfrac{7}{2}\)