To calculate how many landlines will be used in 7 years you have to apply the exponential decay
Where
a is the initial value
r is the decay rate (this value is given as a percentage, you have to use it expressed as a decimal)
x is the time interval that has passed
We know that there are 42000 landlines this year
The declining rate is 10% → expressed as a decimal value r=0.1
The time-lapse is 7 years
In 7 years there will be 20088.47 landlines