Question

Substitution.
x=3
2y+x=3

Answer 1

x = 3
now sub 3 in for x in the other equation

2y + x = 3
2y + 3 = 3 --- subtract 3 from both sides
2y = 0
y = 0

check..
2y + x = 3
2(0) + 3 = 3
3 = 3 (correct)

answer : x = 3 and y = 0
any questions ?

Answer 2

Thank you @texaschic101

Answer 3

no problem :)