Answer:
<em>36 centimeters in the poster</em>
Step-by-step explanation:
Let us plan out our steps, and solve for each:
<em>1. Given the information, let us create a proportionality as such:</em>
3 = 1 ⇒ x - <em>centimeters in the poster</em>
x 12
<em>2. Now let us cross multiply, and solve through simple algebra for x:</em>
3 * 12 = x,
x = <em>36 centimeters in the poster</em>