Find the Coefficient a of the Term in the Expansion $~~~~~~$

Multiple Choice

Find the coefficient a of the term in the expansion of the binomial. Binomial $~~~~~~$ Term
$( 3 x + 2 y ) ^ { 7 }$$~~~~~~~~~~a x ^ { 3 } y ^ { 4 }$

A) a = 11
B) $a = 15,120$
C) a = 21
D) $a = 128$
E) $a = 840$