For (a), you can split the area of the deck into several areas, then sum the areas for the total
You can solve the area of the octagon with side length 1.7 (assuming you're given the formula.) \(\displaystyle 2(1+sqrt(2))a^2\) where a is the side length
You can then solve for the remaining deck as the rectangle of length 5.2 and height 4.0 minus the missing triangular top corner of side lengths 1.2 and 1.7 (as 4.0 - 2.8 = 1.2 and 5.2 -4 = 1.2)
Note: If the triangle were a right angle triangle, the hypotenuse would be slightly less than 1.7. Heron's Formula can be used, though I think (1/2)b*h will be close enough, using 1.2 as base and height.
Add the areas to get the total area.
(b) Solve for area of circle of diameter 2
(c) Area of backyard not covered by deck = Area of backyard - Area of deck