use minimum number of NAND gates to realize this boolean expression
You can walk through the expression using de Morgans, a nand gate can be an inverter, an OR gate or a AND gate by combination, 



image source: https://www.quora.com/How-do-I-convert-logic-gates-by-using-only-NAND-gates

So you walk through, build up the brute force way of making it, then go back over it to see if anything can be optimised to reduce it, e.g. doubled inverters,