If the concentration of the stock solution of Auxins is 0.5mg/ml then how much amount of stock solution is to be added for the preparation of 700ml medium with 2.5mg/L Auxin concentration?

Just put in the values in C₁V₁= C₂V₂where C1 is=0.5mg/ml and V1 is what you have to find out. So lets keep it X C₂ is 2.5mg/L (convert Litres to ml) and V₂= 700ml

0.5 mg/ml * X = 2.5 mg/ 1000 ml * 700 ml
X= 3.5 ml has to be added from stock solution to prepare 700 ml working solution