Find all complex solutions of 3x^2+6x+1=0 . (if there is more than one solution, separate them with commas.)

Use the quadratic formula:
for equation Ax^2+Bx+C=0
x=(-b +/- sqrt(b^2-4ac))/2a
substituting, 
=(6 +/-sqrt(6^2-4*3*1))/(2*3)
=(6 +/- 0)6
=1 multiplicity 2. (both real)
If the complex form is required, then
{1+0i, multiplicity 2}