expanded using pascal's triangle or binomial theorem

You build Pascal's triangle as follows: every row starts and ends with 1, and every term is the sum of the two immediately above. So, we have

We can use this triangle to expand powers as follows: the binomial will feature all the powers , where , with some coefficients: in this case, we have

and the coefficients are exactly the required row of the Pascal's triangle. Removing also unnecessary exponents, we have

Finally, substitute a=2x and b=4y to get the

the answer is 21%.