The range is simply the output.
This is asking for to find the range of values for between x = -2 and for x =2
f(x) = 3x - 12
for x = -2, f(-2) = 3*(-2) - 12 = -6 - 12 = -18
for x = 2, f(2) = 3*(2) - 12 = 6 - 12 = -6
Therefore range of values for f(x) is between -18 and -6
Range is: (-18, -6)