Resposta:

a)

3x² -15 x + 12=0

divida por 3

x²-5x+4=0

x'=[-(-5)+√((-5)²-4*1*4)]/2= [5+√(25-16)]/2=(5+3)/2=4

x''=[-(-5)-√((-5)²-4*1*4)]/2= [5-√(25-16)]/2=(5-3)/2=1

b)

Se for  10x² +75x -64=0

x'=[-(75)+√((75)²-4*10*(-64)]/20=[-75+√(5625+2560]/20

x'=[-75 +√(8185)]20

x''=[-(75)-√((75)²-4*10*(-64)]/20=[-75-√(5625+2560]/20

x''=[-75 -√(8185)]20