$($a$)$

What is the coefficient of x$3$y$4$ in $(3$x$+4$y$)7?$

$($b$)$

What is the coefficient of x$2$y$7$ in $(5$x$$y$)9?$

$($c$)$

What is the coefficient of x$5$y$3$ in $(3$x$4$y$)8?$

$($d$)$

What is the coefficient of x$6$y$1$ in $(2$x$5$y$)7?$