$7-12.$ A beam of rectangular cross section is simply supported at the ends and subject to the single concentrated force shown in Fig. $7-25($a$).$ Determine the shearing stress at a point $3$ cm below the top of the beam at a section $1$ m to the right of the left reaction. Also, determine the maximum shearing stress due to the vertical shear $V=18kN$

$($b$)$

Fig. $7-25$