12-4/5(x+15)=4

And $${\mathtt{x}} = -{\mathtt{5}}$$

Multiply both sides  by 5.

That will get rid of the fraction and it will be easier.  

12-4/5(x+5)=4

12-12-4/5(x+5)=4-12

-4/5(x+5)=-8

4/5(x+5)=8

5x4/5(x+5)=8x5

4(x+5)=40

(x+5)=10

x=10-5

x=5

If you sub x=5 back into the formula you will find it does not work.

You have made a mistake somewhere.       

Yes indeed. You can use the calculator, and for you might see is wrong. I have no offense with the answer, but you tried your best. The answer is -5. Check your work and you can see that the answer is -5. Love your detailed answer.