Hello!My name is Bob. My classroom is not big but nice. There are 21 desks and 20 chairs. 墙上有一些画和一幅地图。Mrs Brown is our English teacher. She is from a big city.She is nice. Her husband Mr Brown is a teacher too, but he isn't in our school. Tom is my good friend. We like playing football on Saturday. Where is my football? It is under my desk.     

①点燃酒精灯；②加热；③用试管夹夹好试管；④往试管中加入2-3mL食盐水；⑤将试剂瓶盖好，放回原处．

( 参考公式：用最小二乘法求线性回归方程系数公式 $\widehat{b}=\frac{{\sum }_{i=1}^n{x}_i{y}_i-n\overline{x}\overline{y}}{{\sum }_{i=1}^n{x}_i^2-n{\overline{x}}^2}，\widehat{a}=\overline{y}-\widehat{b}\overline{x}$ )