Find the constant k such that the quadratic 2x^2 + 3x + 8x - x^2 + 4x + k has a double root.
\(x^2+15x+k\) with a=1,b=15 and c=k we want double root so \(\sqrt{b^2-4ac}=0 <=> \sqrt{15^2-4k}=0 <=> \sqrt{15^2} =\sqrt{4k}=0 <=> k=225/4\)