What is the equation of a circle with a center at (-1,0)?

You have not given enough information for us to complete the full equation for a circle but we can get close. 

The equation of a circle is: (x - h)² + (y - k)² = radius²
where the center is represented by (h , k). I shorted the word 'radius' to variable r.

You would just plug in your given information and simplify:
(x - (- 1))² + (y - 0)² = r²

Your answer would be:
(x + 1)² + y² = r²

You did not provide a radius or another point on the circle so we could find the radius so you are left with this incomplete equation of a circle. 

Don’t know sorry about that