Consider the two vectors A and B .The magnitude of their sumi .e , /A+B/ , if /A/ is greater than /B/

According to question the sum of two vectors is equal to resultant R
and
 $$\begin{gathered} R =\overrightarrow{A} +\overrightarrow{B} \\ \left|R\right| =\left|\overrightarrow{A} +\overrightarrow{B}\right| (taking mod to both sides) \\ {\left|R\right|}^2 ={\left|\overrightarrow{A} +\overrightarrow{B}\right|}^2 \\ R =A^2+B^2+2AB\cos \theta \end{gathered}$$
here $\theta$ is the angle between both vector A and B , but in question  only given 
$\left|\overrightarrow{A}\right| > \left|\overrightarrow{B}\right|$   which just clears that the magnitude of vector A is greater than magnitude of vector B
if we put the numerical values in the  resultant formula we will get the desired result.