Can't get my head around this at all any help would be greatly appreciated.

Answer:
The input is -6, hope this helps :D
Step-by-step explanation:
Represent the machines as expressions:
outputA= 7x-6
outputB= 3x+2
Find the input when the outputA is 3*outputB:
outputA=3outputB
(7x-6)=3(3x+2)
7x-6=9x+6
2x=-12
x=-6
Answer:
[tex]x=-6[/tex]
Step-by-step explanation:
[tex]A=7x-6\\B=3x+2\\For A=3B\\7x-6=3(3x+2)\\7x-6=9x+6\\2x=-12\\x=-6\\[/tex]