3a+5b⋮13⇒2(3a+5b)⋮13or6a+10b⋮13

We have :13a+13b⋮13⇒(13a+13b)−(6a+10b)⋮13

⇒7a+3b⋮13

\(3a+5b⋮13\Rightarrow2\left(3a+5b\right)⋮13\)or\(6a+10b⋮13\)

We have :\(13a+13b⋮13\Rightarrow\left(13a+13b\right)-\left(6a+10b\right)⋮13\)

\(\Rightarrow7a+3b⋮13\)