Chapter 12 - Exercises and Problems - Page 218: 55

$840N$

As the bracket is in equilibrium so $\sum \tau_{pivot}=0$ $\implies F_B\times 4.5-W\times 12=0$ $F_B=\frac{W\times 12}{4.5}=\frac{314\times 12}{4.5}=840N$ We also know that $\sum F_x=0$ $\implies F_{Ax}+F_B=0$ $F_{Ax}=-F_B$ $F_{Ax}=-840N$ The negative sign indicates that the force is in opposite direction.