<span>f '(x) = 2(3x2<span> + 5) + 6x(2x - 1)</span></span><span>= 6x2<span> + 10 + 12x</span>2<span> - 6x</span></span><span>= 18x2<span> - 6x + 10</span></span>
The first step is to name the equations to make the process of solving them easy.

The second step is to multiply each of the equations by the coefficient of the x term in the other equation. In this case, we will multiply equation (1) by 5 to get equation (3) and then multiply equation (2) by 6 to get equation (4).

The next step is to add equation equation(3) and (4) together to get equation (5) as shown below;

The next step is to take the value of y and substitute it into any of the equations above. For this solution, I pick equation (1). The work done to solve for x is shown below.;

The solution for this system of equations is 
Answer:
equation is: width = area / length
width = 62.5 yd
Explanation:
For the first rectangle:
We have:
length = 75 yd and width = 60 yd
Therefore:
area = length * width
area = 75 * 60
area = 4500 yd²
For the second rectangle:
We have:
area = area of first rectangle = 4500 yd²
length = 72 yd
area = length * width
width = area / length ............> The required equation
width = 4500 / 72
width = 62.5 yd
Hope this helps :)