The slope formula is (y2 - y1)/(x2 - x1). Substituting the numbers with the corresponding variables gives (9 - 8)/(-3 - 5). Combining like terms then gives 1/-8. You likely got -8 by inverting the numerator and denominator in the formula so what you did was likely (x2 - x1)/(y2 - y1) instead of (y2 - y1)/(x2 - x1). Just remember that slope is rise/run.
