Well,though not clearly mentioned,considering,the reaction to be,
D ------> D_1 + beta ------> D_2+alphaD−−−−−→D1+β−−−−−→D2+α
Given, mass number of D_2D2 is 176176, now as D_2D2 has been formed from D_1D1 due to alphaα emission,mass number of D_1D1 must be (176+4)-180(176+4)−180 and similarly atomic number should be 71+2=7371+2=73 (as, alpha particle has mass number 44 and atomic number 22)
So, we can complete the 2nd2nd half of the reaction,
""_73^180 D_2 -------> ""_71^176 D_3 + ""_2^4 He18073D2−−−−−−→17671D3+42He
Similarly, we can say, there will be no change in mass number of DD as,beta particle has mass number of 00 as beta emission is expressed as, ""_(-1)^ 0 beta0−1β,So its atomic number must decrease by 11 i.e 7272
So,the whole reaction becomes,
""_72^180 D ---->""_73^180 D_2 + ""_-1^0 beta -------> ""_71^176 D_3 + ""_2^4 He18072D−−−→18073D2+0−1β−−−−−−→17671D3+42He