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