The reaction at A is twice as the reaction at B
$R_A = 2R_B$
$\Sigma F_V = 0$
$R_A + R_B = 600 + 200$
$2R_B + R_B = 800$
$3R_B = 800$
$R_B = 266.67 \, \text{ lb}$
$\Sigma M_A = 0$
$600x + 200(x + 4) = 15R_B$
$600x + 200x + 800 = 15(266.67)$
$800x = 3200$
$x = 4 \, \text{ ft}$ answer