When you are talking about similarity between triangles... you are talking about using a proportion. in this case..
1) ABC ~ DEF, 
2) 5/7 = 16/x, now we are using the variable X because we don't know the value of DE.. 
3) Now, you cross multiply... you should get something like ---> 5x=112
4)divide 5 both sides 
5) you get X=22.4 
so the length of DE is 22.4
I HOPE I HELPED.