Could it be that since the slope is upwards, the acceleration has to have opposite sign of v0? if we choose the + x-axis to be upwards along the slope, then v0 will be positive, while acceleration will slow down and be negative (opposite to v0)?
If so, then s= 3(1.2) + (1/2)(-2.5)(1.2)^2 = 3.6 -...