This is a quadratic, with a=5, b=3 and c=8.
-3 plus or minus sqrt( 9-4(5)(8) )
Roots are x = ---------------------------------------------
2(5)
-3 plus or minus sqrt( 9 - 160)
x = -----------------------------------------
10
-3 plus or minus i*sqrt(151)
= --------------------------------------------
10
Notice that there are 2 distinct roots here.
Let's add them together:
-3 + i*sqrt(151) -3 - i*sqrt(151) -6
-------------------- + ---------------------- = ------- = -3/5 (answer)
10 10 10