Variables & Expressions
Letters that stand in for numbers you don't know yet.
Concept
A variable is a placeholder
In 4x + 7, the x is a slot. Give it a value and the whole expression collapses to a number.
Terms with the identical variable part are 'like terms' and can be combined. 4x + 3x = 7x, but 4x + 7 stays as it is.
ax + bx = (a + b)x