F$=2.[8$PTS$]$ You are given two beams with the cross$-$sentions shown below. Knowing that that the moment of mertia about the centroid of a rectangle is given by:

$I==\frac{1}{12}b{h}^3$

and the following dimensions:

$b_1=10mm,h=18mm,t=3mm$

calculate the value of the width of the second cross$-$section, $b_2,$ that gives both

beams the sime moment of incrtia about the $-$axis $(l_z).$