are you trying to solve for x and y?
y=2x+1; x+2y=17
sub in one variable in the other equation.
x+2(2x+1)=17
x+4x+2=17
5x=15
x=3
now that you have x value, plug into the original equation to find y and check with the other.
y=2(3)+1 = 7
x+2y=17 = 3 + 2(7) = 17 ; 17=17