Factoring, need to understand how to reduce?

First you need to multiply the whole equation by -1 so that the x squared does not have a negative sign (You cannot factorise in this way if the x squared is negative, remember). That would leave you with:

x^2 - 40x - 500 (which is exactly the same as your original equation)
Now you need to find two numbers that multiply to give -500 and add to give -40 : -50 and 10

So your final answer would be:
(x - 50) (x + 10)

+ 50 - 10

-x²+40x+500 = -x² + 50x - 10x + 500
= -x ( x - 50) - 10 ( x - 50)
- (x - 50) ( x-10)

-x^2 + 40x + 500
= -(x^2 - 40x - 500)
= -(x^2 + 10x - 50x - 500)
= -[(x^2 + 10x) - (50x + 500)]
= -[x(x + 10) - 50(x + 10)]
= -(x + 10)(x - 50)

-x²+40x+500 = -x²+50x - 10x+50*10 = -x(x- 50) - 10(x - 50)
= (50 -x)(x+10)

-x^2 + 40x +500= -(x+10)(x-50)

-x^2 + 40x +500=-(x+10)(x-50)=(x+10)(50-x)