You just need to plug those expressions inside the formula: it doesn't matter if they're expressions involving a variable instead of plain numbers: the formula becomes

If you want, you can simplify it by expanding the square and then multiply the two parenthesis:

It is called Ecological Succesion hope that helped:)
Sqr (x2 - x1)^2 + (y2 - y1)^2
Sqr (-4-0)^2 + (2+5)^2
Sqr (16) + 49 = sqr 65
I got around 8.
(I might did something wrong, idk)
Answer:
Read the chapter and concepts First