we are working on square roots. The question askes us to find the indicated function calues for f(x)= square root of 9x+7 then we are given f(1), f(-1), and F(2) can't figure out what to do

f(1)= square root of 9(1)+7= square root of 16=4
f(-1)= square root of 9(-1)+7 = square root of -2 (undefined)
f(2)= square root of 9(2)+7 = square root of 25=5