Since the two triangles are similar, their corresponding sides are proportional. Let's set up a proportion between the corresponding sides of the two triangles to find $a$ and $b$.

Step-by-Step Solution

1. Given information:

   • First triangle (let's call it Triangle 1):
     • Horizontal side = $1.4$
     • Vertical side = $2.1$
   • Second triangle (Triangle 2):
     • Horizontal side = $b$
     • Vertical side = $a$

2. Proportion setup: Since the triangles are similar, the ratios of the corresponding sides are equal: $\frac{1.4}{b} = \frac{2.1}{a}$

3. Simplifying the Ratio: Dividing both 2.1 and 1.4 by 0.7 gives: $\frac{2.1}{1.4} = 1.5$ This means: $\frac{a}{b} = 1.5$

4. Finding $ab$