The functions f(x) and g(x) are described below:

f(x) = 3x − 7
g(x) = 3x + 8

the graph of g(x) is obtained by shifting up the graph of f(x) by units.

One way is this
y=mx+b
b=y intercept

f(x)=3x+7
g(x)=3x-8

f(x) has y intercept at y=7
g(x) has y intercept at y=-8
distance between them is 15 units down

another way
to move a graph c units up, add c to whole function
f(x)=3x+7
to g(x)=3x-8
added -15 to the funtion
moved it up -15 units or down 15 units