Hello Ekta, to calculate Rb you need the value of Vb (i.e the base voltage). The relation is
Vb = Ib(base current) * Rb
=> Rb = Vb/Ib Ohms
Since Vb will be in Volt, Ib will be written as 35 * 10^-6 A
This is simple application of Ohms Law and nothing much difficult.
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