Hey there! I'm happy to help!
We want to get the a all by itself. For me, I always like to have the variable on the left side, so first, let's just flip this around.
[tex]\frac{a-13}{-6} =1[/tex]
What we want to do is get all of the constants (numbers without variables) on the right side. This will leave a all on its own.
We see that the quantity a-13 is being divided by -6. To undo this, we will just multiply both sides by -6. This will cancel out that -6 and get us closer to having the a isolated.
[tex]\frac{a-13}{-6} *-6=1*-6\\\frac{a-13}{1}=-6\\a-13=-6[/tex]
Now we add 13 to both sides to undo the -13 on the left side.
[tex]a-13+13=-6+13[/tex]
[tex]a+0=7\\a=7[/tex]
Have a wonderful day and keep on learning! :D