Hi Suc, the equation for the perimeter of a circle is: Pc = pi * D, where P is the perimeter and D is the diameter.
For your question, The perimeter of a semicircle is: Ps = D + 1/2 * pi * D
Solving this for D gives: D = Ps / (1 + 1/2 * pi)
Substituting for Ps (10.28) and pi (3.14) gives:
D = 10.28 / (1 + 1/2 * 3.14)
D = 3.997 centimeters, or 4.0 centimeters rounded.