The length of a rectangle is 2ft longer then the with. The perimeter is 24ft. Find the dimension?

L = W + 2
2L + 2W = 24

You have two lines. They look like this:
www.wolframalpha.com/input/?i=plot+y%3Dx%2B2,+2y+%2B+2x+%3D+24

Where they cross, the values of x and y satisfy both equations simultaneously, so it is called a simultaneous system. There are two ways to find that point. Substitution is when you solve one equation for one variable and substitute that into the other equation. Since one is already solved for L, we will use that one.

2(W + 2) + 2W = 24
2W + 4 + 2W = 24
4W = 20
W = 5 and L = 7

Elimination is when you add the equations in a way to eliminate one variable. Otherwise the process is the same.

2L + 2W = 24
2L = 2W + 4
------------------- SUBTRACT
2W = -2W + 20
4W = 20
W = 5 and L = 7