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The area of the garden will be as follows,
The Area formula of a rectangle is
  A = length x width
The Area formula of a semicircle is
  A = r²
Join both these formulas
Total Area = Area of rectangle + Area of semicircle
The two sides are 26 ft and 20 ft
Area of rectangle  = 26 x 20 = 520 ft²
Area of semicircle = (10)² {radius is 10 ft since diameter is 20 ft} = (3.14)(10)² = 157 ft²
Total Area = 157 + 520 = 677 ft²