Studiot, I have attempted your modification of the question my methodology is shown below:
In triangle CBO:
θ = tan^-1(0.6)
θ = 30.96375653...
tan(θ) = h/y
y = h/tan(θ)
y = 7.8/tan(30.96375653...)
y = 13
In triangle AOB:
alpha = 23.57818 degrees
beta = 29.55192 degrees
gamma = 126.86990 degrees
a/sin(a) = b/sin(b) = c/sin© [sine rule]
sin(alpha)/y = sin(gamma)/d
d/sin(126.86990) = 13/sin(23.57818)
d = (13/sin(23.57818))*sin(126.86990)
d = 25.99999762
d = 26 (nearest whole number)
Imatfaal, Your proposal matches the answer in the back of the book. Though I'm not familiar with using pi in this topic.