You can write this as two equations, then solve them.
let B be the board length, let T be the total length
so we have...
4B = T+3
3B = T-5
we need to make both equations equal to T, so we can later remove that variable:
T+3 - 3 = 4B - 3
Therefore: T = 4B-3
T-5 + 5 = 3B + 5
Therefore : T = 3B+5
Because they are both equal to T we can make them equal to eachother and reorder the equation to get B on one side:
4B - 3 = 3B + 5
4B - 3 + 3 - 3B = 3B + 5 + 3 - 3B
therefore B = 8
thus we know that the board's length is 8ft long,
Hope this helps! :)
