what is the slope of speed-time graph?

答左你先 , slope of speed-time graph = acceleration

幅圖向上斜 , accelration 就係正 (因為速度增加緊嘛)
幅圖向下斜 , accelration 就係負 (因為速度減少緊)
幅圖平的 , accelration 就係零 , 冇加速 (因為速度不變)

仔細考慮下
slope = 垂直距離 / 水平距離
而speed time graph黎講
slope = speed / time = acceleration
(垂直係speed , 水平係time)

the slope of speed- time graph is the acceleration.
Why?

Slope could be obtained by equation (y2-y1)/(x2-x1)

In the speed time graph the value of Y is speed and X is Time

The rate of change of speed with respected to time is the acceleration

Make it simple, the change of speed in each second we treate as acceleration, right?

hence, the slope at each point is the acceleration.

slope = (y2-y1)/(x2-x1)

in the case of v-t graph,

slope= (v2 - v1)/(t2-t1)

where t2-t1 is the time taken

that's exactly the same with the eqt of acceleration!