The long piece should measure 7.5 feet or 7'6" (7 foot 6 inches).
If x is the length of the short piece then the length of the long piece must be 3x. The total length of the uncut beam is 10 foot.
This can be represented by :
x + 3x = 10
4x = 10
x = 2.5
As x = 2.5 = length of short piece, then the long piece measures 3x = 3(2.5) = 7.5 feet.
There are 12 inches in a foot so 7.5 feet = 7'6"