Algebra  Substitution
"Substitute" means to put in the place of another.
Substitution
In Algebra "Substitution" means putting numbers where the letters are:

If you have: 


And you know that x=6 ... 


... then you can "substitute" 6 for x: 

Example 1: If x=5 and y=3, then what is 10/x + 2y ?
Put "5" where "x" is, and "3" where "y" is: 10/5 + 2×3 = 2 + 6 = 8
Example 2: If x=3 and y=4, then what is x^{2} + xy ?
Put "3" where "x" is, and "4" where "y" is: 3^{2} + 3×4 = 9 + 12 = 21
Example 3: If x=3 (but you don't know "y"), then what is x^{2} + xy ?
Put "3" where "x" is: 3^{2} + 3y = 9 + 3y (that is as far as you can get)
As that last example showed, you may not always get a number for an answer, sometimes just a simpler formula.
