how do you make 50ml of .1 molar solution of benzylamine in acetonitrile
How many moles do you want? That's M x L = ?
Then mols = grams/molar mass. You know molar mass and mols, solve for grams.
Add that many g benzylamine to a 50 mL volumetric flask, add some acetonitrile, swirl until solute is dissolved, make the to 50 mL mark with solvent, mix thoroughly.