1st step: break each of these arrows (vectors) into their x- and y-components. The pedestrian vector would be <6,0>.
The wind vector would be <-13cos(45), 13sin(45)> (Notice the x-component is negative because it is to the left.)
2nd step. Add the two vectors: <6, 0> + <-9.1924,9.1924> = <-3.1924, 9.1924> This is the resultant vector in rectangular coordinates. Now you need the magnitude of this vector.
3rd step: Magnitude is always positive and it is sqrt[x-coord2 + y-coord2]
Magnitude = sqrt(94.6915598949) = 9.7309588374 rounded to whatever
