The expression ( x^2 - 225 ) can be factored using the difference of squares formula, which states that ( a^2 - b^2 = (a-b)(a+b) ). Here, ( x^2 - 225 ) can be rewritten as ( (x - 15)(x + 15) ) since ( 225 ) is ( 15^2 ). This means that the expression equals zero when ( x = 15 ) or ( x = -15 ).