Given #tan(alpha-beta)=7/24 and tanalpha=4/3#
So #(tanalpha-tanbeta)/(1+tanalphatanbeta)=7/24#
#=>(4/3-tanbeta)/(1+4/3tanbeta)=7/24#
#=>32-24tanbeta=7+28/3tanbeta#
#=>100/3tanbeta=25#
#=>tanbeta=3/4#
So #cotalpha =3/4 and cotbeta=4/3#
Now
#cot(alpha+beta)=(cotalphacotbeta-1)/(cotbeta+cotalpha)#
#=>cot(alpha+beta)=(3/4xx4/3-1)/(4/3+3/4)=0=cot(pi/2)#
#=>alpha+beta=pi/2#
proved