We sum vectors by summing the coefficient of corresponding coordinates:
[tex]v = ai+bj,\quad w = ci+dj \implies v+w = (a+c)i+(b+d)j[/tex]
So, in your case, we have
[tex]u+v = (-4+4)i + (1+1)j = 0i+2j = 2j[/tex]
The norm of a vector is the square root of the sum of the squares of its coefficients:
[tex]u = ai+bj \implies ||u||=\sqrt{a^2+b^2}[/tex]
So, in your case, we have
[tex]||u+v|| = ||0i+2j|| = \sqrt{0^2+2^2} = \sqrt{4}=2[/tex]
