=x^2 y^2 -z^2

=(xy+z)(xy-z)

2. Square of (m+n)-4 (m+n)+4

=(m+n-2)^2

3. the square of x (x-y)+the square of y (y-x)

=(x-y)(x^2 -y^2)

=(x-y)(x-y)(x+y)

=(x-y)^2 (x+y)

4. Wrong question, it should be: 49(a-b) squared-16(a+b) squared.

=(7a-7b+4a+4b)(7a-7b-4a-4b)

=( 1 1a-3b)(3a- 1 1b)

5. The square of (the square of x+the square of y)-the square of 4x.

=(x^2 +y^2 -2xy)^2

={(x-y)^2 }^2

=(x-y)^4