[tex]4 {x}^{2} - 5x - 21 = 0[/tex]

D=b²-4ac=(-5)²-4×4×(-21)=25+336=361

[tex]x1 = \frac{ - b + \sqrt{d} }{2a} = \frac{5 + 19}{8} = 3 \\ x2 = \frac{ - b - \sqrt{d} }{2a} = \frac{5 - 19}{8} = - 1.25[/tex]

Ответ: 3; 1,25