Answer
Maximum safe load will be 900 pounds for the asked beam
Work Step by Step
Let the maximum safe load be denoted by $M_{SL}$
width of the beam by W
Thickness of the beam by T
Length by L
Now
$M_{SL} = k\frac{WT^2}{L}$
Given
$M_{SL} = 750$ when $W = 4, T = 2$ and $L = 8$
Putting the value in the equation
$750 = k\frac{4*2^2}{8} => k = 375$
To find $M_{SL}$ when $W = 6, T = 2$ and $L = 10$
Putting the value n the equation we get
$M_{SL} = 375\frac{6*2^2}{10} = 900$
Maximum safe load will be 900 pounds for the asked beam
