First identify the quantities involved
let L be length of the long piece
let S be length of each shorter piece (notice there are two of them of the same length)
second, Translate the words into equations using those quantities.
Translate “47 feet of molding in three pieces” into “47 ft = L + S + S”
Translate “long piece is 2 feet longer than three times the length of the shorter piece” into “L = 2 + 3S”
now you you have two equations in two unknowns. You probably already know how to solve them. (I‘d substitute the expression for L into the first equation, and then solve for S. Then substitute that value in and solve for L.)