13. Determine the values of c for which x^2 + 2cx - 4 will have exactly 2 real roots.
Apply the following:
For the quadratic $\displaystyle Ax^2 + Bx + C$ the discriminant is defined to be $\displaystyle \Delta = B^2 - 4AC$.
$\displaystyle \Delta > 0 \, :$ The quadratic has 2 real roots
.
$\displaystyle \Delta = 0 \, :$ The quadratic has 1 (repeated) root.
$\displaystyle \Delta < 0 \, :$ The quadratic has no real roots.
Do not confuse the c in your question with the C in the above formulae.