Since you are adding A and -B (not regular B), you would draw a copy of B starting from the tip of A, but facing in the opposite direction as before (with the arrow pointing towards the third quadrant). Then draw an arrow from the origin to the end of -B to see what your resultant vector, A-B, looks like.

To find the magnitude, you need to split both vectors A and -B into components. Remember that you add the x-components of both vectors together to get the x-component of A-B, and you add the y-components of A and -B together to get the y-component of A-B.

The x-component is magnitude*cos(theta), where theta is the angle with respect to the x-axis. Since A lies along the x-axis, its theta is 0 degrees. B has a theta of 36 degrees, but you want -B. From your picture you can see that you can get -B by rotating B a certain number of degrees. Can you figure out what the theta for -B is?

The y-component is magnitude*sin(theta). Once you have all the components, add the appropriate ones to get the final x-component and final y-component of A-B.

Finally, remember that the magnitude is found by pretending that the components are the sides of a right triangle. So we use the Pythagorean Theorem to get the magnitude of the resultant vector. Magnitude^2 = (x-component)^2 + (y-component)^2