Ineed will give brainliest if answer is correct

So, the one you picked is correct because:




This eliminates the second option because there is only 1 possible value for a. 

The 4th option is also correct because it's saying:


Now, just by pure logic, you can tell that to solve for b, you have to use option 3 because Option 5 has already been used to solve for a.

So, your answers are: #1, #3, and #4.

The answer to that is 6