If it is a parallelogram, then AC = BD. Therefore we have the equation:
5x + 4 = 3x + 8 subtract 4 from both sides
5x = 3x + 4 subtract 3x from both sides
2x = 4 divide both sides by 2
x = 2
BD = 3x + 8
Put the value of x:
BD = 3(2) + 8 = 6 + 8 = 14
Answer: 14