Value of variable given others in formulae

Question

u=20, t=1.2, k=-5

 

D= ut + kr²

 

(a) Work out the value of D.

 

The answer I got for this is 16.8 which is most likely incorrect because I have never learnt how to answer this question so I used my knowledge of other things and tried it, I know for part b you have to rearrange formulae which i cant do so help is very appreciated.

 

(b) D = 50

      t = 5

      k = -5

 

-Work out the value of u (for the formula I wrote above part a) 

 

Apologies I have written this in quite a confusing way 

Answer 1

What is confusing is that you appear to believe that you'll get a numerical answer for D but you have only supplied numerical values of u, t and k, omitting a value for r.

 

On the other hand, if the question were phrased "solve for r in terms of the other variables and their values" that could be done.  YOU MUST LEARN THIS TYPE OF SKILL. Do not accept your present shortcoming because it will be a severe obstacle!

 

To solve your formula for r, first subtract ut from both sides. Then divide out k, the coefficient of r. Lastly, take square root; the result is r = √( (D - ut) / k).  What you have done is to undo operations, in the reverse order, to isolate r.