A metal plate (k = 180 w/m·k, rho = 2800 kg/m³, and cp = 880 j/kg·k) with a thickness of 1 cm is being cooled by air at 5°c with a convection heat transfer coefficient of 30 w/m²·k. if the initial temperature of the plate is 300°c, determine the plate temperature gradient at the surface after 128 seconds of cooling. (round the final answer to one decimal place.)