We can resolve the vectors into horizontal and vertical components (x and y).
Angle 305° is -55° for the purpose of calculation.
a[x]=31cos(-55); b[x]=24cos(70); note that cos(-55)=cos(55) and sin(-55)=-sin(55).
a[y]=31sin(-55); b[y]=24sin(70).
The vector, c, 3a+b can be written in component form:
c[x]=93cos(55)+24cos(70)=61.5511 approx
c[y]=-93sin(55)+24sin[70]=-53.6285 approx.
|c|=√(c[x])²+(c[y])²=81.6367N approx.
tanθ=c[y]/c[x]=-0.8713 approx making θ=-41.07° approx=318.93°.
The resultant vector 3a+b=81.64N@318.93°.