The area of a rectangle is it's width multiplied by it's length.
For this rectangle, it's lenght is:
[tex]L=x+5[/tex]
And it's width is
[tex]W=x[/tex]
When you multiply both of them you get the area A:
[tex]A=L\cdot W=(x+5)\cdot x[/tex]
It can also be writen as:
[tex]A=x^2+5x[/tex]
