What value in place of the question mark makes the polynomial below a perfect square trinomial x2 plus 26 x plus?

To form a perfect square trinomial from the expression (x^2 + 26x + ?), we need to find the constant that completes the square. The formula for a perfect square trinomial is ((x + a)^2 = x^2 + 2ax + a^2). Here, (2a = 26) gives (a = 13), so (a^2 = 169). Therefore, the value that replaces the question mark is (169).