One place to start is to use the basic quadratic formula:
With a=1, b=-10, and c=k, and letting x=5 (as one of the roots) gives you k=25.
Substituting this back into the question gives you x^2-10x+25=0. This is easily factorised to give x=5, twice.
So the answer to the question is that the other solution is also 5.
you can use discriminant X(squared)-11x+k=aX(squared)+bx+c
a=1, b=-11, c=k, and let's say the discriminant D
solutions are s1 and s2
now you put everywhere by squared and you get
so now let's look for S2
the second root is -16