You mean diagonally?
If he has to walk north then west to go back, it will be 9 + 12 yards which is 21 yards.
If not we can use Pythagorean theorem for this.
a^2 * b^2 = c^2
c is longest side
a and b are the other two sides
or simply a and b will be yards that Darren walked
and c is the shortest distance.
9^2 * 12^2 = c^2
81 * 144 = c^2
225 = c^2
in order to cancel the square in the c, we need to get the square root of 225 and c^2
√225 = √c^2
so we have
15 = c
Darren can walk 15 yards as the shortest distance from his stopping point back to the house.
I hope it helps.