how is it possible to find market equilibrium of a perfectly competitive firm given the shape of its market demand curve(horizontal straight line)

The answer to this question is already covered in our study material. Follow the given link and scroll down to the topic ' When Demand is perfectly inelastic and Supply Changes'

https://www.meritnation.com/cbse/class12-commerce/studymaterial/economics/introductory-microeconomics/market-equilibrium/338_1984_5822#slide2_market-equilibrium-under-fixed-number-of-firms