First, let's expand the terms:

(5x-2y)(3x+5y) = 15x^2 + 25xy - 6xy - 10y^2
= 15x^2 + 19xy - 10y^2

(2.5x-3y)(4x+8y) = 10x^2 + 20xy - 12.5xy - 24y^2
= 10x^2 + 7.5xy - 24y^2

Now, let's subtract the second expanded expression from the first one:

(15x^2 + 19xy - 10y^2) - (10x^2 + 7.5xy - 24y^2)
= 15x^2 + 19xy - 10y^2 - 10x^2 - 7.5xy + 24y^2
= 5x^2 + 11.5xy + 14y^2

Therefore, (5x-2y)(3x+5y)-(2.5x-3y)(4x+8y) = 5x^2 + 11.5xy + 14y^2.