At the start, the temperature T is 72. This initial temperature decreases (cools down) by 3 degrees every hour.
In math language, “T is 72” translates to “T = 72”.
“Beginning to cool,” suggests a variation but “T = 72” is the initial stage. The variation is introduced by cooling or lowering of temperature, which means subtraction from the initial stage.
The variation is introduced by “h” hours x 3 degrees/hr, or 3h.
The equation T = 72 – 3h describes this.
It is also the only formula that solves the temperature correctly when h=0, or at the start when no time has elapsed yet.