Its kind of hard to explain but if i showed you my picture that i drew it would be easy to understand

Let’s say that the length of the 4 boards = x

Let’s say that the length of the 3 boards = y

Then 7L = 4x +3y

Where x=L + 3 and y=L-5

If you substitute for x and y , the equation will not agree on both sides because one of the boards comes 3 ft shorter than L. So we have to write

7L-3 = 4x + 3y

In other words, the 4X3 =12 feet you save from the 4 boards is shorter than 3×5 =15 feet you need to make up on the 3 boards. So you will have a porch with one board 3 ft shorter than its length.

4X=3X+5+3

4X=3X+8

4X-3X=8

X=8

Each board’s length is 8 feet.

Here’s how you can check it:

4(8)=32 and the problem says that it’s 3 ft too long so…32-3=29 feet.

3(8)=24 and the problem says that it’s 5 ft too short so…24+5=29 feet. These two lengths are the same; therefore, each board’s length is 8 feet and by plugging in the #’s again, the table’s 29 feet.

12 = -15

= 27