Which value of b would make x^2 - bx +81 a perfect square trinomial?

Data for the height of a model rocket launch in the air over time are shown in the table. darryl predicted that the height of the rocket at 8 seconds would be about 93 meters. did darryl make a good prediction? yes, the data are best modeled by a quadratic function. the value of the function at 8 seconds is about 93 meters. yes, the data are best modeled by a linear function. the first differences are constant, so about 93 meters at 8 seconds is reasonable. no, the data are exponential. a better prediction is 150 meters. no, the data are best modeled by a cubic function. a better prediction is 100 meters.