Find the quadratic equation whose roots are twice the roots of 4x^2+8x-5=0.

ax^2+bx+c
4x^2+8x-5=0 
since a*c=-20 
we use 10 and -2 as follows 

4x^2+10x-2x-5=0 
2x(2x+5)-1(2x+5)=0 
(2x-1)(2x+5)=0 
so x=1/2 or x=-5/2 
and so 
(x-1)(x+5)=0 
x^2-6x-5=0 

:)