I don’t understand these

So these are basically isolating the variables.
The first equation is 3g + 5 =17.
In order to isolate the variable, we would have to get g by itself, that means 5 would have to go. In order to do this, we would do the opposite. Since it is positive 5 we would add negative 5, in order for it to disappear. This works because a positive 5 and negative 5 cancel each other out. Whatever you do to one side of the equation you have to do to the other, since we subtract 5 on one side we have to subtract 5 on the other. Therefore we would do 17-5.
Now we have 3g=12
We know that 3g is basically 3 multiplied by g. The opposite of multiplication is division.Therefore we would divide by 3 on both sides.
The answer to the first question would be g= 4.
And if you want to check if your answer is correct you plug the value in.
So
3(4) + 5 =17