2x^3 -7x +3=0, use quadratic eq:

x= ( 7+- sqrt( 49 -4(2)(3)) )/4

= ( 7 +- 5)/4 then

x = 12/4=3 or x = 2/4= 1/2

3z^2 + 6z = -3

3z^2 + 6z +3 =0

z^3 + 2z + 1 = ( z+1)^2 =0 => there is only 1 solution z = -1, but

you can also use the quadratic formula , if the discriminant b^2 -4ac>0 then there are 2 different solutions, if b^2 -4ac =0 then there is 1 solution

and if b^2 -4ac <0 then there are no solutions at all

For instance, you can’t add x and x^2 together and come up with 2x^2. You CAN add 2x^2 + 3x^2 to get 5x^2 however.

So, you can see there is nothing in your equations that you can add together.