we know that
the area of the garden is equal to the area of a t-r-a-p-e-z-o-i-d
[tex]A=\frac{1}{2}*(L1+L2)*h[/tex]
where
L1 and L2 are the parallel bases
h is the height between the two parallel bases
in this problem we have
[tex]L1=20\ m\\L2=15\ m\\h=12\ m[/tex]
substitute the values in the formula
[tex]A=\frac{1}{2}*(20+15)*12[/tex]
[tex]A=210\ m^{2}[/tex]
therefore
the answer is
[tex]A=210\ m^{2}[/tex]
