Solve by completing the square:

-2x^2+24x-40=0

To start this, we want the equation x²+bx+c=0, so we can start by making the x². This would mean that we can divide the whole equation by - 2 to get it, resulting in x²-12x+20=0. Next, a formula we have is to, first, divide the - 12 by 2, resulting in - 6 and (x-6) ² due to that to get into the equation x²-12x, we have (x-a) ²=x²-2ax+a². As - 12=2a, a=-6. After that, we have to see what the result is and adjust based off of that. As we're squaring (x-6) ², we get x²-12x+36, not just x²-12x, so we have to add 36 to both sides to get (x-6) ²+20=36. Subtracting 20 from both sides, we get (x-6) ²=16. Square rooting both sides, we get x-6=+-√16=+-4. Note that we have the plus or minus due to that for something squared to be a certain number, that something can be positive or negative, e. g. 1² = (-1) ²=1. Therefore, if we add 6 to both sides, we get either 10 or 2.

Feel free to ask further questions!