i think
This is your typical algebra question where you are looking for an unknown value given some information. The information can be set up as:
8 - x = 4*(x +3)
Distribute the 4:
8 - x = 4x + 12
Rearrange to get variables on one side:
8 - 12 = 4x + x
Simplifiy:
-4 = 5x
Isolate the variable:
-4/5 = x