Remember that
in a parallelogram, the diagonals bisect each other
that means
VZ=ZX
substitute given values
n+7=3n
solve for n
3n-n=7
2n=7
n=3.5
therefore
VX=(n+7)+3n ----> by addition segments postulate
VX=4n+7
substitute the value of n
VX=4(3.5)+7
VX=21
