cos(a,b)=a*b/|a|*|b|=-1/2
a*b=-1/2*|a|*|b|=-1/2*1*2=-1
|2a+b|=√(2a+b)^2=√(4a^2+4ab+b^2)
=√(4|a|^2+4ab+|b|^2)
=√(4*1+4*(-1)+2^2)
=2