What is the difference between a constant and a coefficient? when can you add or subtract coefficients? explain

A constant is a value that never changes, like 2, it is always worth 2, so it has a constant value.
A coefficient is a number that multiplies a variable like 3x, 100m is the coefficient.

you can only add or subtract them when they are the same value like 3k+87k=90k it is important to add the letter and when it is like 3y+6+6x+5y=8y+6+6x you only combine like terms.