Answer
We can not construct a triangle with the values given in this question.
Work Step by Step
We can use the law of sines to find the angle $A$:
$\frac{b}{sin~B} = \frac{a}{sin~A}$
$sin~A = \frac{a~sin~B}{b}$
$sin~A = \frac{(859~m)~sin~(74.3^{\circ})}{783~m}$
$sin~A = 1.056$
Since there is no angle A such that $sin~A \gt 1$, the angle A is not defined. We can not construct a triangle with the values given in this question.