Design black box test suit for :-(software engineering question)

i)a program that accepts two strings and checks if the first string is a substring of the second string and displays the number of times the first string occurs in the second string.Assume that each of the two strings has less size than twenty characters

ii)a program that accepts a pair of points defining a staright line and another point and a float number defining the center of a circle and its radius.The program is intended to compute their points of intersection and prints them