Hi there! The answer is k = 16.
Let's solve this problem step by step!
[tex]64 \div k = 4[/tex]
Multiply both sides by k.
[tex]64 = 4k[/tex]
Switch sides. 
[tex]4k = 64[/tex]
Divide both sides by 4.
[tex]k =  \frac{64}{4}  = 16[/tex]
~ Hope this helps you!