Answer:
784 in² of wood
Step-by-step explanation:
Sarah  needs two supports by shelf, and  she will build 4 shelves
then she will need 8 supports
Now each support is a  right isosceles triangle.
Area of a triangle is
A(t)  = 1/2 *b*h    where  b is the base and h is the heigh of the triangle.
A right isosceles triangle has two sides ( base and heigh equals) and in our particular case are equals to 14 in, therefore
A(t) Â = 1/2*14*14
A(t)  =  98 in²
Sarah needs 8 pieces of 98 in² she needs
A = Â 8*98
A  = 784 in² of wood