a.\(9.\left(x+5\right)^2-\left(x-7\right)^2\)

\(=3^2.\left(x+5\right)^2-\left(x-7\right)^2\)

\(=\left(x+8\right)^2-\left(x-7\right)^2\)

\(=\left(x+8-x+7\right)\left(x+8+x-7\right)\)

\(=15\left(2x+1\right)\)

\(=30x+15\)

b.\(49\left(y-4\right)^2-9\left(y-2\right)^2\)

\(=7^2.\left(y-4\right)^2-3^2\left(y-2\right)^2\)

\(=\left(y+3\right)^2-\left(y+1\right)^2\)

\(=\left(y+3-y-1\right)\left(y+3+y+1\right)\)

\(=2\left(2y+4\right)\)

\(=4y+8\)