g(13) means the value of function g when x is 13.
Function g maps values from the orange oval to the blue oval.
x = 13 is the bottom value in the orange oval.
When you use x = 13 as an input in function g, the output is 16 shown in the blue oval.
Function f maps values from the blue oval to the green oval.
Look at the bottom value in the blue oval. It is 16.
When you use function f to map values from the blue oval to the green oval, 16 is mapped to 32.
f(16) = 32.
f(g(13)) means first find g(13).
We found that g(13) = 16.
Now use 16 as x in function f, and find f(16).
We see that f(16) = 32
f(g(13)) = f(16) = 32
f(16) = 32