Your answer (at the point where (s,t) = (0,1)) is correct. To find the value of x, just put s=0 and t=1 in the original equations. You get
After factorising as and using the given information that x>0, you should be able to solve those equations to get x=2. That gives the answer as , not –16 as the book claims. It looks as though the book is wrong.