Answer
$-560x^{4}y^{3} $
Work Step by Step
$n$th term in
$\left( a+b\right) ^{k}$ is $\binom k {n-1} a^{k+1-n}b^{n-1}$
So 4th term in $\left( 2x-y\right) ^{7}\Rightarrow \binom 7 3 \left( 2x\right) ^{4}\left( -y\right) ^{3}=-\dfrac {7!}{3!\left( 7-3\right) !}\times 16x^{4}y^{3}=-560x^{4}y^{3} $
