The discriminant value of a quadratic equation is b^2 - 4ac.
We can see here, in this quadratic equation, 25 is a, 40 is b and 16 is c (as in ax^2 + bx + c)
Applying it to the discriminant value = 40^2 - 4 x 25 x 16 = 1600 - 1600 = 0.
(And since the discriminant value is 0, the quadratic function has 1 repeated real root.)