For this, you'll be subtracting the area of the garden plot from the area of the whole backyard. However, you need to convert feet to yards. I'll be using dimensional analysis to solve that:
[tex] \frac{2 ft}{1} *\frac{12in}{1ft} *\frac{1yd}{36in} =\frac{24}{36} =\frac{2}{3} yd [/tex]
Now that we know its 2/3 yd, we can solve for the areas.
Rectangle: [tex] A=bh \\A=\frac{3}{1} *\frac{2}{3} \\ \\ A=\frac{6}{3} =2 [/tex]
Triangle: [tex] A=\frac{1}{2} bh\\ \\A=\frac{1}{2} *6*8\\\\ A=24 [/tex]
Now that we got the areas, its time to subtract: [tex] 24-2=22 [/tex]
The grass will be 22 yds^2