If two triangles have bases $a$ and $b$ and the same height $h$, the ratio of their areas $A:B$ equals—

The area of a triangle is $\frac{1}{2}\times\text{base}\times\text{height}$, so with equal heights the areas are proportional to the bases. Thus $A:B=\frac{1}{2}ah:\frac{1}{2}bh$.