Awindow washer has a 15-foot extension ladder. how far away from the building should the foot of the ladder be placed to reach 12 feet up the building?

This is the concept of application of  geometry; the distance that the ladder should be placed will be given by Pythagorean theorem.
thus;
c^2=a^2+b^2
where, c=hypotenuse, a and b are the legs
hence;
15^2+12^2+b^2
b^2=15^2-12^2
b^2=225-144
b^2=81
getting the square root of both sides;
b=sqrt81
b=9
therefore the ladder should be placed 9 feet from the wall for it to reach 12 feet up the building

Idon't take stuff like this new to me