Form a recurrence tree of this equation with root as βn.....\
at each level u will get cost as βn and height of the tree will be (log β+log n)/logα
so (βn+βn+βn.......up to above hight will be....
=βn*(logβ+logn)/log α
=(βnlogβ)/logα +(βnlogn)/logαΞΘ(nlogn)