Question 54940
This problem has three variables and therefore requires three equations.  The variables are the price of PCs which we will call P, the price of CD players which we will call C and the price of TVs which we will call T.

The equations are the following taken directly from the text.
2P+3C+4T=250
3P+1C+2T=190
5P+2C+1T=280

You must solve two equations for the other two,  Since one of the variables equals one in two of the equations, let's solve for those.

The first equation is
3P+1C+2T=190             subtract 3P from both sides
1C+2T=190-3P             subtract 2T from both sides
1C=190-3P-2T             Done

The second equation is
5P+2C+1T=280             subtract 5P from both sides
2C+1T=280-5P             subtract 2C from both sides
1T=280=5P-2C             Done

Put the first equation into the second.
T=280-2C-5P              Put in first equation answer
T=280-2(190-2T-3P)-5P    Multiply out
T=280-380+4T+6P-5P       Simplify
T=-100+4T+1P             add 100 to both sides
T+100=4T+1P              subtract 4T to both sides
T-4T+100=1P              simplify
-3T+100=P

Put this answer into the first equation.
C=190-2T-3P              substitute
C=190-2T-3(100-3T)       Mulitply
C=190-2T-300+9T          simplify
C=-110+7T

Put these two answers into an original equation.
2P+3C+4T=250             Original equation now insert results
2(-3T+100)+3(-110+7T)+4T=250  Multiply out
-6T+200-330-21T+4T=250        add 330 to both sides
-6T+200-21T+4T=250+330        subtrcat 200 from both sides
-6T-21T+4T=380                simiplify
19T=380                       divide by 19
T=20                          Done

Plug T into equations
P=100-3T
P=100-3(20)
P=100-60
P=40

C=-110+7T
C=-110+7(20)
C=-110+140
C=30

Now Check you answers:
2(40)+3(30)+4(20)=250
80+90+80=250

3(40)+1(30)+2(20)=190
120+30+40=190

5(40)+2(30)+1(20)=280
200+60+20=280