T is greater than -15 because numbers after -15, like -16... are less than -15, so when its decreasing it gets lower. Since the questions says the temp stayed above -15, that means the temp could be -14...-13 and so on.
okay so you can use the variable "T" for temp, so T stayed above, meaning higher than, -15 to convert this to an equation, use the greater than sign (>)