Hey there bro!
To start this all out,

We are going to use to following:
![\left[\begin{array}{ccc}\boxed{\boxed{A= (a+b)/(2)h}} \end{array}\right]](https://img.qammunity.org/2019/formulas/mathematics/middle-school/e87qoumlitubopmx5uefegodaxxn3x688m.png)
We would then,

By doing this, our sum of these would give us
. . . .
![\left[\begin{array}{ccc}(350\end{array}\right]](https://img.qammunity.org/2019/formulas/mathematics/middle-school/edwn4xvhcrgrphlfv4r00thgkb40r71m5a.png)
SO, IF THE PAINT ONLY COVERS (200) SQUARE FEET, AND 350 WOULD ALMOST BE THE DOUBLE OF 200, WE WOULD THEN UNDERSTAND MORE ON HOW TO FIND SHE WOULD NED TO BUY FOR THE PATIO.
We would then do
![\left[\begin{array}{ccc}(12/4)\end{array}\right]](https://img.qammunity.org/2019/formulas/mathematics/middle-school/rdrqmlojieovlnlv5w77iqc1g90uogwev3.png)
because this is (350) and then from here, we would know how much more she would need.
So, if she only uses ($12.00) for 200 square feet, then we do.
($12.00) + ($4.00) =

dollars.
She would only need to spend
($16.00) bucks.
Hope this helps you bro! :)