Let the cost price of the watch be \(x\) rupees.
According to the question, since the watch was sold for ₹336, the profit percent is equal to the cost price, i.e., \(x\%\).
\(\therefore\) Profit on ₹100 is \(x\) rupees
So, profit on ₹\(x\) is \(\cfrac{x}{100} \times x = \cfrac{x^2}{100}\) rupees
\(\therefore\) According to the question, \(x + \cfrac{x^2}{100} = 336\)
Or, \(\cfrac{100x + x^2}{100} = 336\)
Or, \(100x + x^2 = 33600\)
Or, \(x^2 + 100x - 33600 = 0\)
\(\therefore\) The required quadratic equation is \(x^2 + 100x - 33600 = 0\)