We have been given that the garden and the walkway around its perimeter have an area of 378 square feet.
Let us suppose the width of the walkway is x feet. Please see the attached image.
From the attached image, the area of the garden and the walkway around its perimeter is given by
![A=(15+2x)(12+2x)\\ \\ A=4x^2+54x+180](https://img.qammunity.org/2019/formulas/mathematics/high-school/a5nyi6gmw9mwfmh12pzhkk0o78l2d1irbw.png)
Now, the area is given bt 378 square feet. Hence, we have
![4x^2+54x+180=378\\ \\ 4x^2+54x - 198=0\\ \\ \text{Using the quadratic formula, we have}\\ \\ x_(1,\:2)=(-54\pm √(54^2-4\cdot \:4\left(-198\right)))/(2\cdot \:4)\\ \\ x_(1,2)=3,-(33)/(2)\\ \\ \text{Width can't be negative}\\ \\ \text{Hence, we have} \\ x=3](https://img.qammunity.org/2019/formulas/mathematics/high-school/ek97z2cned135ellmyzqe7jc67mh2b01ay.png)
Therefore, the width of walkway is 3 feet.