Answer:
y=x/2*3
Or, in LaTex, $y = \frac {x}{2} \cdot 3$
If you do this the regular way, you can see that there are three groups of 2 inches in the widths and six groups of 2 inches in the length(dividing by 2 inches and taking the unit out). Multiplying the number of groups by 3 feet, you get 9 feet as the width and 18 feet as the length, which makes sense since the drawing was at a 1:2 ratio for width to length.
But if you make this into an equation, consider the steps;
1) Divide by 2 (inches)
2) Multiply by 3 (feet)
And that's about it. So your equation would be;
y=x/2*3