Exact sequences of semistable vector bundles on algebraic curves

Let X be a smooth complex projective curve of genus g greater than or equal to 1. If g greater than or equal to 2, then assume further that X is eithe r bielliptic or with general moduli. Fix integers r,s, a, b with r > 1, s > 1 and as less than or equal to br. Here we prove the existence of an exact sequence 0 --> H --> E --> Q --> 0 of semistable vector bundles on X with rk(H) = r, rk(Q) = s, deg(H)= a and deg(Q)= b.