Resposta :
a= -1
b= 10
c= -21
delta = 10² - 4 . -1. -21 = 100 - 84 = 16
x1 = (- 10 + raiz de 16 )/ -2 = (-10 + 4)/-2= -6/-2= 3
x2 = ( -10 - raiz quadrada de 16)/-2 = -14/-2 = 7
[tex]-\text{x}^2+10\text{x}-21=0[/tex]
[tex]\text{x}=\dfrac{-10\pm\sqrt{10^2-4\cdot(-1)\cdot(-21)}}{2\cdot(-1)}=\dfrac{-10\pm4}{-2}[/tex]
[tex]\text{x}_1=\dfrac{-10+4}{-2}=3[/tex]
[tex]\text{x}_2=\dfrac{-10-4}{-2}=7[/tex]