Using Maximize[-a*x^2-4*x+5,x] define a function which for given number a value returns  x value. (2p) 
Plot the graf of f(a) for a∈{1,5}, AxesLabel, Thick, Blue (1p)

Q=((2*Pi*f*L)/R),f=(1/(2*Pi*(Sqrt[L*c]))) , find Using Solve vaues L and c. (2p)
Evaluate L*c (1b)

Using NDSolve solve  {x1'(t) == sin(x2(t)), x2'(t) == -tanh(x1(t)), x2(0) == 0, x1(0) == 1}, t∈(0,20).(2b)
Using ParametricPlot plot {x1[t], x2[t]} pro t∈(0,20), AxesLabel, Thick, Red. (2p)

Import file http://www.powerwiki.cz/attach/MAA_EN/test.csv  (1p)
The file contains lists {t,x1,x2,x3}, Using ListPlot plot x3 as a function of parameter t (2p)
Find the maximal value of x3 for t∈(0,0.3) and display the result using Print (2p)