Question says that the graph of function g is a vertical stretch of the graph of function f by a factor of 3.
Now we have to find about which equation describes function g.
We know that if f(x) is the given function then a*f(x) gives vertical stretch or compress by a factor of a.
when 0<a<1 then it compresses by factor of a.
when a>1 then it stretches vertically by factor of a.
Given that function f is stretched by a factor of a so that means we use a=3 in formula a*f(x)
Hence final answer will be g(x)=3f(x).
So choice A is correct answer.
