a. The gym has the shape of a rectangle.
The area of a rectangle is calculated as follows:
![A=\text{base}\cdot\text{height}](https://img.qammunity.org/2023/formulas/mathematics/high-school/mr8em9u8vfxwzcyse9spso5a4867f7kaq2.png)
The base of the gym is x and its height is 15, then its area is:
![\begin{gathered} A=x\cdot15 \\ A=15x \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/emnqh7ppqqloyp1fgfbxoejknifqf3tm4h.png)
b. The cafeteria is a rectangle with a height of 15 and a base of (x+3), then its area is:
![\begin{gathered} A=(x+3)\cdot15 \\ \text{Distributing:} \\ A=x\cdot15+3\cdot15 \\ A=15x+45 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/7xegu7bgyoerj8f3n1ll5zohzoji8mugml.png)
c. The total area is the addition of the gym area and the cafeteria area.
![\begin{gathered} A=15x+15x+45 \\ \text{Combining similar terms} \\ A=30x+45 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/iyt2ies6194m2pbkxbyciuqbk3dlasyqf7.png)
d. The perimeter of a rectangle is calculated as follows:
![P=2(base+height)](https://img.qammunity.org/2023/formulas/mathematics/high-school/sdj93itcw7l0icejnfunkocbthwh8ff3h6.png)
Considering both the gym and the cafeteria, the base is x + x + 3 = 2x + 3 units long. The height is 15, then:
![\begin{gathered} P=2(2x+3+15) \\ P=2(2x+18) \\ P=2\cdot2x+2\cdot18 \\ P=4x+36 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/5t7sye0qbce0eqjvw5rdvs4l8c5vl468gc.png)