What is the line of symmetry for the parabola whose equation is y = x2 + 10x + 25

In order to do this, use the simple formula x = -b/2a, where a and b are taken directly from the equation for the parabola.  a = 1 and b = 10, so your line of symmetry equation is x = -10/2(1) or x = -5, which is also the x coordinate of the vertex. The long way that will tell you that y coordinate of the vertex would be to put it into vertex form by completing the square.  But not necessary here as you are only being asked for the line of symmetry.  If you are told to find the coordinates of the vertex this simplified equation will not work.