5,6 = 0,4 is not a valid equation as these are just two sets of numbers.
3х+12 = 3 can be solved by rearranging the equation to isolate x:
Subtract 12 from both sides:
3x + 12 - 12 = 3 - 12
3x = -9
Divide by 3:
3x/3 = -9/3
x = -3