Answer:
And replacing we got:

Step-by-step explanation:
Let X the random variable of interest, on this case we now that:
The probability mass function for the Binomial distribution is given as:
Where (nCx) means combinatory and it's given by this formula:
And we can find this probability:

And we can solve this probability like this:

And if we use the probability mass function we got:
And replacing we got:
