Answer:
The length of the longer rope piece is 14 ft
Explanation:
Let
x -----> the longer piece
y ----> the shorter piece
we know that
The sum of the longer piece plus the shorter piece must be equal to the length of the original piece
x+y=21
y=(21-x) -----> equation A
3x-7=5y ----> equation B
substitute equation A in equation B and solve for x
therefore
The length of the longer rope piece is 14 ft