Substitution is a method where one or more variables in an expression is replaced by numbers or another set of variables.To evaluate an expression specific numbers are used as substitute. To express in terms of another set of variables, variables are replaced by the new set of variables.
If x=3, then 2x2+4x+1 can be evaluated by substituting 3 for x.
Replacing all x by 3, we get 2*32+4*3+1=31.
If x=2+y, then x2-3 can be expressed in terms of y by replacing all x by (2+y):(2+y)2-3=y2+4y+1.
Do not forget to replace all occurrences of the variable.
After substitution the variable does not appear anywhere in the expression.