Question 470578
The ratio we can write as:

l/b=8/5

it is given that lenght is 10.5 cm more than breadth  then l=b+10.5, 
now substitute for l
{{{(b+10.5)/b=8/5}}}

8b=5(b+10.5)
3b=52.5
b=17.5

then l=b+10.5=17.5+10.5=28