A formula is a general rule or relation, written as an algebraic equation. There are all types of formulae for calculating various quantities in many subject areas.
Substitution
Substitution involves putting given values into a formula in place of the variables (written as letters).
e.g. is the formula for the area of a trapezium.
If a = 5 cm, b = 7 cm and h = 10 cm
A = 60 cm^{2}
Changing the Subject
The subject of a formula is the variable or letter that stands alone.
e.g. 
A is the subject 

x + y + z = V 
V is the subject 
To make a different variable the subject of a formula, the usual algebraic techniques can be used.
Example 1
To make b the subject of
Example 2
To make x the subject of A = x^{2}
Take the square root of both sides
x = ±√A
Checking your answers
When you have solved an equation, you can always check your answers by substituting the values back into the equation.
(a) Solve x^{2} − 4x − 5 = 0
Factorise to (x − 5)(x + 1) = 0
Either: x − 5 = 0 or x + 1 = 0
x = 5 or x = 1
To check:
x^{2} − 4x − 5 = 0 5^{2} − 4.5 − 5 = 0 25 − 20 − 5 = 0 0 = 0 (This shows the solution x = 5 is correct.) 

x^{2} − 4x − 5 = 0 (1)^{2} − 4.(1) − 5 = 0 1 + 4 − 5 = 0 0 = 0 (This shows the solution x = 1 is correct.) 