Refer to pascal's triangle. draw a circle around the element denoted by:

'6 choose 4' is how it's read
=
so

====15

that means the coeficient of the 5th term is 15

or remember that

is for a binomial to the nth power, to find the k+1 term
so
4+1=5
look at row coresponding to 6th power (7th row from top)
cound 5 from left to right
get 15
15(a)^1(b)^5  is 5th term in binomial (a+b)^6