So to simplify a radical, use the product rule of radicals, which is √ab = √a x √b. And also remember that √a x √a = a.
With these things in mind, you can simplify this radical as such (also remember that the minus sign = -1):
-1√80 = -1√(10 x 8) = -1√(5 x 2 x 4 x 2) = 2 x 2 x -1√5 = -4√5
In short, the simplified radical of -√80 is -4√5.
