We have the following triangle:
the area of this triangle is the following:
![\begin{gathered} A=(b\cdot h)/(2) \\ \Rightarrow A=(3.5\cdot7)/(2)=(24.5)/(2)=12.25 \\ A=12.25\operatorname{cm}^2 \end{gathered}]()
Since Senora Cruz sill use four triangles on the door decoration, we have that the area of the original triangle will be multiplied by 4:
![4\cdot A=4\cdot12.25=49\operatorname{cm}^2]()
Thus, senora Cruz will use 49 square centimeters for the triangles