Sorry to disturb, but could we have found the magnitude of the resultant by using the parallelogram law?
Yes, you can.
The magnitude of the sum of wo vectors ##\vec c= \vec a+\vec b## is
##|c|=\sqrt{ ( \vec a + \vec b)^2}=\sqrt {|a|^2 + |b|^2 + 2|a||b| \cos (θ) }##
which is the length of the "long" diagonal of the parallelogram made by vectors ##\vec a## and ##\vec b##. θ is the angle between the vectors. You can also apply the cosine law for the blue triangle.