Rewrite: 16m²+40m+25=0. Take a guess that this is a perfect square because 16m²=(4m)² and 25=5². If it is then the middle term will have the coefficient 2×4×5=40, which it does! So the equation becomes (4m+5)²=0, and 4m+5=0 (taking the square root of each side), so 4m=-5, m=-5/4.
