Ok, well first find the distances between the offshore platform and both of the other two stations.

Then, use that information and the fact that you know the line that connects the two pipelines will form an angle of 90 degrees.

Notice that the two triangles formed when you draw the line to the onshore pipeline have a common side. Let's call this side x.

So, let's find an equation for this side using both triangles:

Triangle 1: $\displaystyle \sin {a} = \frac {x}{d_1}$ , where $\displaystyle d_1$ is the distance from the offshore platform to the refinery.

Triangle 2: $\displaystyle \sin {b} = \frac {x}{d_2}$ , where $\displaystyle d_2$ is the distance from the offshore platform to the harbor.

Have you calculated the two distances yet?