if w=4/5l and a=320 then subsitute 320 for a dn 4/5l for w 320=4/5l times l multiply both sides by 5 to clear fraction 1600=4l times l ad like terms 1600=4l^2 subtract 1600 from both sides 0=4l^2-1600 try and factor because if you have xy=0 then x and/or y=0 so difference of 2 perfect squares where if you have a^2-b^2 then that equals(a-b)(a+b) so 0=(2l)^2-(40)^2=(2l-40)(2l+40)
so 2l-40=0 add 40 to both sides 2l=40 divide by 2 l=20
if 2l+40=0 subtract 40 from both sides 2l=-40 divide 2 l=-20 since meausrements of real objects cannot be negative, wediscard this number
l=20 is thhe true legnth s subsitute w=4/5l w=4/5(20) w=80/5 w=16
