Not able to find the solution for the following question?

take 7 common

then

7(1+11+111+....................upto n terms)

multiply and divide by 9

7/9(9+99+999+................... nterms)

7/9[(10-1)+(100-1)+(1000-1)+.................. nterms)

applying geometric series sum formula for n terms

7/9[10(10^n-1/9)-n]

7/81[10^(n+1)-9n-10]