Remark
I'm going to interpret this equation as
D = ut + kt² The only difference is the 2.
Solution
Subtract kt² from both sides.
D - kt² = ut Now divide by u on both sides.
[tex]\dfrac{D - kt^2}{t} = \dfrac{ut}{t}[/tex]
The t's cancel out. on the right. You are left with u on the right.
[tex]\dfrac{D - kt^2}{t} =\text{u}[/tex]
