Question 153470
There are many ways to solve a quadratic polynomials For me, unless the problem requires a specific method OR unless the equation is simple enough to 'factor' in my head, I prefer to use the quadratic equation.

{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
Just plug in -16 for a, 21 for b and 40 for c.

Another way to solve, if you have a graphing calculator, is to plot the graph and then see where the the plot crosses the x axis. 

In either case, you get the same answers

*[invoke quadratic "x", -16, 21, 40]