the slope formula is the following
[tex]y=m\mleft(x-x_0\mright)+y_0[/tex]
where m can be calculated as
[tex]m=\frac{y_1-y_0}{_{}x_1-x_0}[/tex]
where
(x0,y0) = (3,12) and
(x1,y1) = (5,18)
From the above, we can replace and obtain the following:
[tex]\begin{gathered} y=\frac{y_1-y_0}{_{}x_1-x_0}(x-x_0)+y_0 \\ y=\frac{18-12}{5-3}(x-3)+12 \\ y=\frac{6}{2}(x-3)+12 \\ y=3(x-3)+12 \\ y=3x-9+12 \\ y=3x+3 \end{gathered}[/tex]
