You should not multiply the height by just one base. Instead, combine the two base lengths in your formula before using the height.

First add the bases, then multiply by $\tfrac{1}{2}$, and finally multiply by the height. Be careful not to skip the $\tfrac{1}{2}$ factor.

In Desmos, type the expression ((12 + 18)/2)*10 to represent $\frac{1}{2}(12+18)\cdot 10$, the area formula with the given values.

Look at the value Desmos outputs for this expression; that number is the area of the trapezoid in square centimeters and matches one of the answer choices.

For a trapezoid with bases $b_1$ and $b_2$ and height $h$, the area $A$ is given by

This is the same as "average of the bases times the height."

Here, the bases are 12 cm and 18 cm, and the height is 10 cm. So,

First add the bases: $12 + 18 = 30$. So,

Now multiply:

• Half of 30 is $\frac{1}{2} \cdot 30 = 15$.
• Then $15 \cdot 10 = 150$.

So the area of the trapezoid is 150 square centimeters, which corresponds to choice B.