Question

A number is multipled by its double and the answer is 72. What is the number ?

Answer 1

$x \times 2x = 72 \\ 2x^{2} = 72 \\ \frac{2x^{2} }{2} = \frac{72}{2} \\ {x}^{2} = 36 \\ \sqrt{ {x}^{2} } = 36 \\ x = 6$

Answer 2

Let x be the number

Then,
x(2x) = 72
2x^2 = 72
x^2 = 36
x = ±√36
x = ±6

The number could be 6 and -6.