If one is given the distance between the sources of light (perpendicular to the line of sight) and the distance between the sources and the lens, there is a formula in terms of a tangent. But here you are in luck, they give you the value of theta in the problem!
They are 1.1 arcsecond apart (if they said second, they really meant arcsecond). Do you know what an arc second is? It is one degree divided by 3600 (this comes from the fact that one degree is 60 arcminutes and one arcminute is 60 arcsenonds). So your angle is 1.1/3600 degrees. Now convert that in radians, plug that in the equation and you are done.