To simplify the expression, first expand the square of (2x - 3y):

(2x - 3y)^2 = (2x - 3y)(2x - 3y)
= 4x^2 - 6xy - 6xy + 9y^2
= 4x^2 - 12xy + 9y^2

Now we substitute this into the original expression:

(10x - 15y) : (4x^2 - 12xy + 9y^2) : 2x

Now divide (10x - 15y) by 2x:

= (5x - 7.5y) / (4x^2 - 12xy + 9y^2)

Therefore, the simplified expression is (5x - 7.5y) / (4x^2 - 12xy + 9y^2).