This formula represents the situation of taking out two balls from six balls, and it is a combination formula in the arrangement and combination of middle school mathematics.

The definition and calculation formula of combination: from n different elements, any m(m≤n) elements are grouped, which is called the combination of m elements of n different elements; The number of all combinations of m(m≤n) elements from n different elements is called the number of combinations of m elements from n different elements. Represented by the symbol C(n, m). C(n，m)=A(n，m)/m！ Where m! Represents the factorial of m, m! = m * (m-1) (m-2) ...1,I hope I can help you.