$\Sigma M_{R4} = 0$
$12R_3 = 4 [ \, \frac{1}{2}(12)(400) \, ]$
$R_3 = 800 \, \text{ lb}$
$\Sigma M_{R3} = 0$
$12R_4 = 8 [ \, \frac{1}{2}(12)(400) \, ]$
$R_4 = 1600 \, \text{ lb}$
$\Sigma M_{R2} = 0$
$16R_1 = 13(1600) + 12R_3$
$16R_1 = 13(1600) + 12(800)$
$R_1 = 1900 \, \text{ lb}$ answer
$\Sigma M_{R1} = 0$
$16R_2 = 3(1600) + 4R_3 + 16R_4$
$16R_2 = 3(1600) + 4(800) + 16(1600)$
$R_2 = 2100 \, \text{ lb}$ answer