The following determinants are obtained from the simultaneous equations in variables x and y.
`D_(x)|{:(-11,a),(9,-4):}|,D_(y)=|{:(3,-11),(b,9):}|,D=|{:(3,2),(7,-4):}|`
The solutions of the equations are x=-1 and y=-4. Find the values of a and b. Also find the original simultaneous equations having this solution.