ClearAll["Global`*"] (* First side of Class #4 worksheet *)
(* Repeat the Same Problem doing just "Solve" *)
R1=20.0;R2=30.0;R3=15.0;R4=25.0;R5=10.0;R6=20.0;
V0=10.0;i0=0.03; (* in Amps, not mA! *)
(* start by writing all 12 equations using == *)
equations = {V2-V1==i1 R1,V1-V4== i2 R2, V3-V2== i3 R3,V4-V3==i4 R4, V2-0== i5 R5, V3-0 == i6 R6, V1== V3 + V0,i1+i7==i2,i3==i1+i5,i2+i0==i4,i4 == i6+i3+i7};
variables = {V1,V2,V3,V4,i1,i2,i3,i4,i5,i6,i7};
Solve[equations, variables][[1]]

These are the same results we got with the matrix methods. 
Having Mathematica makes this a little easier... you don’t need a “matrix”.
Most companies don’t use Mathematica, so after you graduate, “Solve” will not be an option. 
You’ll have to either do the 11 equation algebra by hand or with a Matrix, since lots of software (even Excel!) can do matrices.