What is the perimeter of a square abcd if a(0,0 and b(0,5?

 The perimeter of this square is:  " 20 units " .

Explanation:

A square has all 4 sides of equal length.  From the information given, we know that one side length has "5 units".  Thus, all the side lengths are "5 units".

       The formula/equation for the perimeter of a rectangle (which would include a square; since ALL squares are rectangles):

   P = 2L + 2w ; 
                      in which:  P = perimeter; 
                               L = length = 5 units; 
                               w = width  = 5 units;

So we plug in our known values; to solve for the perimeter, "P" ;

         P = 2*5  + 2*5  = 10 + 10 = 20 units.

Alternately,  since this is a square, and we know that a square has all four side of equal length (and we know that each side length of this square is "5 units";  and the "perimeter" equals the sum of the lengths of all the sides:

We can calculate the "perimeter" as follows:  5 units * 4 (sides) = 20 units.
       Or, perimeter = 5 + 5 + 5 + 5 = 20 units.

 The perimeter of this square is:  " 20 units " .
 

Could you make it more clear?