Можно по-разному...
1) D=14²-4*5*q = 196-20q
x₁ = (14-√(196-20q))/10
x₂ = (14+√(196-20q))/10
x₂ > x₁
x₂ = 2.5x₁
(14+√(196-20q))/10 = 2.5*(14-√(196-20q))/10
14+√(196-20q) = 35-2.5√(196-20q)
-21 = -3.5√(196-20q)
√(196-20q) = 6
196-20q = 36
20q = 160
q = 8
------------------- можно по т.Виета...
2) 5x² - 14x + q = 0 ---> x² - 2.8x +(q/5) = 0
{ x₁*x₂ = q/5
{ x₁+x₂ = 2.8
-----------------
{ x₁ * 2.5x₁ = q/5
{ x₁ + 2.5x₁ = 2.8
-----------------------
{ 12.5*(x₁)² = q
{ 3.5 * x₁ = 2.8
x₁ = 28/35 = 4/5 = 0.8
q = 12.5 * 16 / 25 = 16*0.5 = 8