Question 107119
Let a=x+3, b=x, c=15 
a^2+b^2=c^2
(x+3)^2+x^2=15^2
x^2+6x+9+x^2-15^2=0
2x^2+6x+9-225=0
2x^2+6x-216
2(x^2+3x-108)=0
2(x+12)(x-9)=0
x= -12 is not an answer (negative)
x= 9 ft from from the building.
Check:
81+144=225 true
Ed