Call the length [tex]l[/tex] and the width [tex]w[/tex]
We are told:
1. "The length of a rectangle is 3 times its width." i.e.: [tex]l=3w[/tex]
2. "The rectangle’s width is 4 m." i.e.: [tex]w=4[/tex]
So, by substitution we have:
[tex]l=3w[/tex]
[tex]l=3(4)[/tex]
[tex]l=12m[/tex]
Then we can use our area formula:
[tex]A=lw[/tex]
[tex]A=(12m)(4m)[/tex]
[tex]Area=48m^{2}[/tex]
