Ok. You want to find how much it costs to drill to a depth of 500m. In the question, you are given how much the costs increase, in regards to the multiples of 50m. We know that when a further 50m is drilled, it costs an extra £140. So d = 140.
Think of this like an arithmetic progression, we know that the first term (when a depth of 50m is drilled) equals £500. So within the equation, a = 500.
We want to know how much it will cost when we drill to a depth of 500m. Think back to the question - we need to know what n is (ie which term it is in the sequence) - n = 500 /50 = 10.
Now try and solve it!