Answer
The sailboat gives a speed of about $7\frac{1}{2}\,\text{knots}$ at a $90{}^\circ $ angle to the wind.
Work Step by Step
Let us express the given situation in polar form where each point $\left( r,\theta \right)$ on the graph gives the sailing speed, r, in knots, at an angle $\theta $ to the $10$ knot wind.
From the graph, we can observe that the sailboat gives a speed of about $7\frac{1}{2}$ knots at a $90{}^\circ $ angle to the wind.
