I apologize for the title, I'm not sure what category this would fall under.

The advertisement read Buy $3$ tires at the regular price and get a fourth tire for only $3$ dollars. Carol paid $\$240$ for a set of $4$ tires. What is the regular price of a tire?

So I came up with $3x + 3 = 240$.

$\frac{240}{3} = 80$

$3 = 80$ does not make sense.

Why do I have to subtract the $3$ first? I was told that when using systems of equations I needed to use PEMDAS.