(3x+4y)2 is the product of two binomial expressons, which have the same terms (3x+4y) and (3x+4y). It can be expanded by the method of multiplying a binomial by a binomial. On Comparing the identities with this product. In this product a=3x and b=4y. We can get the result of this product by substituting 3x and 4y terms in the place of a and b respectively in the first identity (a+b)2=a2+2ab+b2
Hence, (3x+4y)2=(3x)2+2(3x)(4y)+4y2
=9x2+24xy+16y2