4x + 3y - z
This is an EXPRESSION. It does no have an an = sign. The parts which are seperated by + or - signs are called terms.
3n - 1 = 17
This is an EQUATION. This equation only has one letter in it. You can solve an equation to find the value of the letter.
A = 1/2bh
This is a FORMULA. You can use it to calculate one value if you know the other values. You can't solve formulas.