Me explain why d has the solution x=-3 you

The best way to solve these types of equations is to plug the solution in and see if it checks out.
1/2x+8=10
1/2*-3+8=10
-1.5+8=10
6.5 is not equal to 10
1/2(2*-3-6)=-6
1/2(-6-6)=-6
1/2*(-12)=-6
-6=-6
so d checks out