Answer
$15.453, \ or, \ -0.453$
Work Step by Step
For the equation $x^2 -15x-7= 0$, we use the quadratic formula as follows
$$
x=\frac{-b \pm \sqrt{b^{2}-4 a c}}{2 a},
\\
a=1, \quad b=-15, \quad c=-7
.$$
That is, $$ x=\frac{15\pm \sqrt{225+28}}{2(1) }=15.453, \ or, \ -0.453
.$$
