Question

5 x square - 16 X - 21 factorise​

Answer 1

Answer:

$5 {x}^{2} - 16x - 21 \\ = 5 {x}^{2} - (21 - 5)x - 21 \\ = 5 {x}^{2} - 21x + 5x - 21 \\ = x(5x - 21) + 1(5x - 21) \\ = (5x - 21)(x + 1)$

It's your answer....

Answer 2

Answer:

$5 {x}^{2} - 16x - 21 = 0$

$a = 5 \: \: b = - 16 \: \: c = - 21$

${b}^{2} - 4ac = 256 - 4 \times 5 \times - 21 = 676$

$x = \frac{ - b + \sqrt{676} }{2a} = \frac{16 + 26}{10} = \frac{42}{10} = \frac{21}{5}$

$x = \frac{21}{5}$

$x = \frac{ - b - \sqrt{676} }{2a} = \frac{ 16 - 26}{10} = \frac{ - 10}{10} = - 1$

$x = - 1$

that's is your answer