What is the equation of a line having slope - 1/3 and y-intercept equal to 6?

Answer: Option A

$$\begin{array}{rl}& \left(y-y_1\right)=\left(\frac{y_2-y_1}{x_2-x_1}\right)\left(x-x_1\right)\\ & \Rightarrow \left(y-6\right)=-\frac{1}{3}\times \left(x\right)\\ & \Rightarrow 3y-18=-x\\ & \Rightarrow x+3y=18\end{array}$$