You have to deal with this type of question step by step. Firstly, you can figure out all the unknown values (such as sqrt x-3), by substituting x for 19, as you are trying to figure out f(x) when x=19. Therefore:
x+2=21
sqrt(x-3)=sqrt(19-3)=sqrt 16=4
Now the equation is easier to handle:
3/21 - 4
We can write this as 3/21 - 4/1 , which equals -27/7.