Question 443613
Get it in the form ax^2 + bx = c

{{{16x^2 + 40x = -21}}}
{{{16(x^2 + expr(5/2)x) = -21}}}
{{{(x^2+expr(5/2)x) = -21/16}}}
Now it is time to complete the square:
Recall that {{{ c  = (b/2)^2}}} 
So  {{{(5/4)^2 = 25/16}}}
Remember to balance the equation by adding what we added to complete the square to both sides.
{{{(x^2 + 5x + expr(25/16)) = expr(-21/16) + 25/16}}}
{{{(x+expr(5/4))^2 = 4/16}}}
{{{highlight((x+expr(5/4))^2 = expr(1/4))}}}