Answer:
1.3m/s²
Step-by-step explanation:
Given values in the first part: acceleration a, time t₁:
1) velocity v₀
![= at_1](https://img.qammunity.org/2020/formulas/physics/high-school/49ag7yosjr0715dw1qxd0cu693soahp59g.png)
2) height h₀
![=(1)/(2)at_1^2](https://img.qammunity.org/2020/formulas/physics/high-school/d5pxwdbjkdzsiomwnkqk2yih4umesqzn85.png)
Given values in the second part: acceleration -g, time t₂:
3) height h
![= -(1)/(2)gt_2^2+v_0t_2+h_0](https://img.qammunity.org/2020/formulas/physics/high-school/jtq7lrlx6apkebe9hvmlonka02fvcdds6f.png)
Combining equations 1,2,3 and setting h to zero:
![0=-(1)/(2)gt_2^2+(at_1)t_2+(1)/(2)at_1^2\\ 0=a(t_1t_2+(1)/(2)t_1^2)-(1)/(2)gt_2^2](https://img.qammunity.org/2020/formulas/physics/high-school/yfcxlomqlat0llqok3vlzkvy9sk7ks109h.png)
Solve for a with t₁ = 4s and t₂=2.1s:
![a=(1)/(2)gt_2^2((1)/(t_1t_2+(1)/(2)t_1^2))](https://img.qammunity.org/2020/formulas/physics/high-school/c2ge1l87e3nan7mmxtt6hx4f5lj0govvzm.png)