Answer:
a) He used the machine on the right
b) 4
Explanation:
a) whatever number you insert on the left machine (y=x²+3), we will always get a number even if you put 0. but on the right machine (y=sqrt•x -2) we have a value to make the output 0
b) if you insert 4 on the right machine,
y=sqrt•4-2
y=2-2
y=0