Equivalent algebraic expressions

Two expressions are equivalent if they are always equal, regardless of which numbers are substituted for the pronumerals.

For example a + 2 is equivalent to 2 + a.

By substituting different numbers for the pronumerals we can see whether two expression are equivalent.

For example let's see if the three following expressions are equivalent;

 3a + 5, 2a + 6 and 7a + 5 - 4a