x=-7
We can write them both like functions like this
A= 5x -4
B= 3x +8
Where
x=input
A=Output A
B=Output B
We want the output of A 3 times the output of B this can be written like this:
A= 3b
we replace with the functions
5x-4 = 3(3x+8)
solve for x and you get -7