Calculate delta S = +70 - (131 + 0.5 x 205) = -163.5 J K-1 mol-1
Then, delta G = delta H - T delta S = -286 - 298 x -163.5 / 1000 = -237.3 kJ mol-1
You need to convert the T delta S term into kJ mol-1 before the addition / subtraction to get a meaningful answer.