Hello, the_rookie07!
I don't suppose you made a sketch . . .
Prove analytically that the diagonals of a square are of equal length,
bisect each other, and are at right angles. Code:

D(0,a)*      *C(a,a)
 
 
 
 
 
  *      *  
A(0,0) B(a,0)
Use the Distance Formula to compare the lengths of and
Write the equation of diagonals and
. . and locate their intersection,
Then use the Distance Formula to show that: . and
You already have the equations of and
. . Simply show that they are perpendicular.