sub 789 for x

-2.5x^2 +2825x-115,812.5
having troubble undeerstanding the process . could someone show me step by step..

first step, as I have shown you on two of these, is do the substitution. Wherever there is an x, replace it with 789:

-2.5(789^2) + 2825(789) - 115812.5

Now just pull out your handy calculator, or use an online calculator, and discover that the value is

556810