Simplification of the expression (-2x+8y)(9x-3y)

You multiply each term by each term when expanding double brackets, so:
-2x × 9x = -18x^2
-2x × -3y = 6xy

8y × 9x = 72xy
8y × -3y = -24y^2

Now you combine the like terms to get
-18x^2 + 78xy - 24y^2
I hope this helps!