Answer:
Diagonal(D) of a square is given by:

where, a is the length of one side of the square.
As per the statement:
Ashley draws a square that has a diagonal of 15 centimeters
⇒

then;

Squaring both sides we have;

Divide both sides by 2 we have;

or

⇒

Therefore, the length of one side of the square (Rounded to the nearest tenth) is, 10.60 cm