I'll bet that your inclusion of units for the coefficients has some folks confused (they might think them to be variables) ...
simple quadratic ...
11 - 1.5t = -12 - 2t + .85t<sup>2</sup>
collect like terms and set = 0 ...
0 = -23 - .5t + .85t<sup>2</sup>
now ... use the quadratic formula to solve for t
a = .85, b = -.5, c = -23