12x28=336
12x32=384
336+384=720 square^2
To make a square for x²+bx=c, add(

)²on both side
in this case, b is-3/4, half of b is -3/8, and (-3/8)² is 9/64, so you add 9/64 to both side.
<span>2.1m = 25.62
divide both sides by 2.1
</span>2.1m / 2.1 = 25.62 / 2.1
simplify
m = 12