Question 1026474
lets follow your question up by expansion as suggested by pascal using pascals triangle.
 In a coin,we have tail (T) and head (H).
Therefore a coin =(H+T)
since there are four coin here,we raise (H+T) to power of 4
That is,(H+T)^4
Expand the above,we have
(H+T)^4=H^4+4H^3T+6H²T²
           +HT^3+H^4
Two head lies on 6H²T²
but p(H)=1/2 also, p(T)=1/2
Therefore,
p(two head)=p(6H²T²)
              =6(1/2)²(1/2)²
  =6(1/4)(1/4)
  =6(1/16)
  =6/16
=3/8
Hope this helps too?