If the value of the discriminant of a quadratic equation $6x^{2}-x-k=0$ is $25$,then $k = \ldots$

If the following quadratic equation has two equal and real roots,then find the value of $k$: $x^{2} - kx + 25 = 0$.

The speed of a motorboat in still water is $25 \text{ km/hr}$. In a river, it covers $60 \text{ km}$ downstream and the same distance upstream in a total time of $5 \text{ hours}$. Find the speed of the current of the river. (The speed of the current of the river is less than the speed of the motorboat in still water.) (in $\text{ km/hr}$)

The symbol used for the discriminant of a quadratic equation is $\ldots \ldots \ldots \ldots .$

Find the roots of the following quadratic equation by using the quadratic formula,if they exist: $2 y^{2}+5 y-3=0$.

The formula for finding the value of $D$ (discriminant) for a quadratic equation $ax^2 + bx + c = 0$ is $\ldots \ldots \ldots \ldots.$