Answer:
Part A) The system of equations is equal to


Part B)
The length is 
The width is 
Step-by-step explanation:
Part A)
we know that
The perimeter of the triangle is equal to the sum of the length of its three sides
In this problem

-----> equation A
The perimeter of rectangle is equal to

In this problem

------> equation B
Part B)
Using a graphing tool
Solve the system of equations
We know that
The intersection point both graphs is the solution of the system of equations


The intersection point is 
see the attached figure


step 3
Find the length and width of the rectangle
The length is 3x

The width is 7y
