Substitution

Substitution, in algebra, means replacing a letter or variable with a number.

The value of the expression can then be worked out. (Sometimes called evaluating).

Sometimes an expression may have two or more variables and each one of these can be given a value.

Expression
Variables
Substitute
Value of expression
3p
p = 5
3 x 5
15
6a + b
a = 2 and b = -3
6 x 2 + -3
9
p(q + 3)
p = 4 and q = 2
4(2 + 3)
20
4n2
n = 2
4 x 22
16

Remember the normal rules of BEDMAS apply to algebra too. This is the ORDER that things are worked out.

Brackets
Exponents
Division
Multiplication
Addition
Subtraction