the complete question in the attached figure
we know that
the formula for combinations is
C=n!/[(n-r)!*r!]
where
n is the total number of objets
r is the number that you choose to arrange
in this problem
n=6
r=4
C=6!/[(6-4)!*4!]-----> C=(6*5)*4!/[2!*4!]----> C=30/2----> C=15
the answer is
15