I think Question no: 5.b need theta value to solve for horizontal range . could you please clarify?

Safna, thanks for the question. This is a unit check problem. The units of range are meters. The units on the right side of the students equation are m/s times the sqr root of m^2/s^2. Multiply them together and get units of m^2/s^2, which are not the units of range!  John