Question 130474
You can't.  Since you have 2 variables (x and y), you need 2 equations in order to solve by substitution (or to solve the system in general).  If you had another equation, such as x+y=1, you could solve by substitution.  So I will solve the system I just made up of -x+2y=17 and x+y=1.  Substitution means solving one equation for x or y, and then plugging that new "x" or "y" into the other equation.  So for my system, I could take the second equation, x+y=1, and solve for x: x=1-y.  Now anywhere I have an x, I can put 1-y.  So the first equation, -x+2y=17, becomes -(1-y)+2y=17.  Distribute the -1 to get -1+y+2y=17.  Simplify to get -1+3y=17.  Add 1 to both sides: 3y=18.  Divide both sides by 3: y=6.  Now plug that value of y back into either of the equations to solve for x.  x+y=1, so x+(6)=1, so x=-5.  So the solution to this system is x=-5, y=6.  Those values work in both equations in the system.