1) The perimeter is the sum of the lengths of the straight edge (the diameter of the semicircle) and the length of the arc of the semicircle.
The circumference of a 4 ft circle is
π*diameter = π*4 ft ≈ 12.566 ft
The semicircle will have a length that is half that, 6.283 ft. When this length is added to the diameter, the perimeter is found to be
Perimeter = 4 ft + 6.283 ft ≈ 10.3 ft.
2) The area of a circle is given by the formula
A = (π/4)d²
For a diameter of 15 inches, the area is
A = (π/4)(15 in)² = 56.25π in²
A ≈ 176.7146 in²
The area of the circle is about 176.71 in².