Factorise \( \left(5 m^{2}-2 n\right)\left(2 m^{2}+4 m n-7 n^{2}\right) \)

To factorise the expression \( \left(5 m^{2}-2 n\right)\left(2 m^{2}+4 m n-7 n^{2}\right) \), we can observe the two factors separately. The first factor, \( 5 m^{2}-2 n \), is already in its simplest form as it can't be factored further without special conditions or integer values for \( m \) and \( n \). The second factor, \( 2 m^{2}+4 m n-7 n^{2} \), can be approached using the method of splitting the middle term or applying the quadratic formula for roots, but given the degree of complexity, it may work better as is for most scenarios unless specific factorable patterns can be observed. Thus, the expression remains as is: \[ \left(5 m^{2}-2 n\right)\left(2 m^{2}+4 m n-7 n^{2}\right). \]