Add 1 to both sides:

In cases like this, we have to remember that a root is always positive, so we can square both sides only assuming that

Under this assumption, we square both sides and we have

The solutions to this equation are

But since we can only accept solutions greater than -1, we discard
and accept
.
In fact, we have

and

which is the only solution.
In your equation: a+(-4)=-13
you then add four to each side,
so a= negative 13 plus 4
so a= -9
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