first of all, your "answer" is incorrect ... moving the foot of the laddercloserto the wall will make the angle between the ladder and the ground increase to a value larger than the initial 70 degrees.

let = initial distance the foot of the ladder is from the wall (in meters)

let = length of the ladder

= new angle between the ladder and the ground

three trig ratio equations can be set up ...

... solve for

you should get