F(4) if f(x) = 3x - 1

Simply substitute the value of 3 given. In other words,

f(3) is the function of

f(x), however, in this case, x is 3

f(3)=3(3)−4

Distribute 3 through the parenthesis

f(3)=9−4

Simplify

f(3)=5

for finding the midpoint of any given 2 pair of points simply add the respective x coordinates with other x coordinate and the y coordinate with other y coordinate and divide them by 2

in your question it will be :

(3+1)/2 for the x coordinate of mid point i.e 2

[8+(-4)]/2 for the y coordinate of midpoint i.e 2

so coordinates of mid point of points (3,8) and (1,-4) will be (2,2)