The total area of the four walls of her bedroom is given by:
[tex]\begin{gathered} A=2\cdot L\cdot H+2\cdot W\cdot H \\ A=2\cdot10\cdot7+2\cdot6\cdot7 \\ A=224\text{ ft}^3 \end{gathered}[/tex]Then, the total cost of the paint is given by:
[tex]0.75\cdot A=0.75\cdot224=\text{\$}168[/tex]