Finding formula of an area question.

Pizzeria Buonapetito makes a triangular-shaped pizza with base width of 18 inches and height 12 inches as shown. Alice wants only a portion of the pizza, so she makes a vertical cut through the pizza and takes the shaded portion. Letting x be the bottom length of Alice's portion and y be the length of the cut as shown, answer the following questions.
Here is a picture of the details above:
Attachment 25173
Attachment 25174


Here are the problems:
Attachment 25175

I'm having a lot of trouble trying to find a formula for her area for the second interval.
How would I approach these problems? Thank you!

for the second triangle (6 < x < 18) ...

shaded area = area of large triangle - area of unshaded smaller triangle