How to find 3P of Elliptic curve when slope is a decimal?

Question

Hi I was soliving Elliptic Curves and I found out $2P$ but when it came to $3P$ I became a little bit stumped. My equation is $$y^2 =x^3 + 2x + 9 \qquad(\text{mod } 23)\ .$$ The values I got for $P$ is $(0,3)$ and $2P$ is $(18,14)$, I do know that when $P\ne Q$ the equation to find the slope for $R$ is $$\frac{(y_2 - y_1)}{(x_2-x_1)}\qquad (\text{mod } 23)\ .$$ However when I try to find the slope I get a decimal number, not sure if I'm missing a step or if I'm supposed to continue as is.