Question 647495
The formula for distance is simply (maybe not simply) 

Where (x1,y1) and (x2,y2) are just two points.

{{{sqrt((x2-x1)^2 + (y2-y1)^2)}}}

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If you understand the distance formula you can skip this.

This formula is NOT random.

This formula pieces together slope and the Pythagorean Theorem.

Recall that for slope  m = (change in y)/(change in x) = (y2-y1)/(x2-x1).

Now, visualize the distance from y1 to y2 and x1 to x2 as sides of a right triangle. x1 to x2 would be side a and y1 to y2 would be side b. The distance would be our hypotenuse, c. Let d be distance. So (c=d).

a^2 + b^2 = c^2

(x2-x1)^2 + (y2-y1)^2 = c^2

{{{sqrt((x2-x1)^2 + (y2-y1)^2) = c}}}

{{{sqrt((x2-x1)^2 + (y2-y1)^2) = d}}}

Now onto your problem.
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(8,5) and (0,20) are our points.

x1 = 8
y1 = 5
x2 = 0
y2 = 20

{{{sqrt((0-8)^2 + (20-5)^2)}}} 
{{{sqrt((-8)^2 + 15^2)}}}
{{{sqrt(64 + 225)}}}
{{{sqrt(289)}}}
{{{d = 17}}}