из формулы длины прямоугольника а= (Р- 2b) / 2 , выразите b - ширину прямоугольника

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The usual meals in England are: breakfast, lunch, tea and dinner, or in simpler houses, breakfast, dinner, tea and supper. In England mealtimes are as follows: breakfast time is between 7 and 9 a.m., lunch time is between 12 and 2 p.m., dinner is between 7 and 10 p.m.

     Usual breakfast is porridge, bacon and eggs, marmalade with buttered toast and tea or coffee. For a change you can have a boiled egg, cold ham or perhaps fish.
     Englishmen generally have lunch at one o'clock p.m. The businessmen usually find it impossible to come for lunch and so they go to a cafe or a restaurant. Afternoon tea you can hardly call a meal, but for some people it has become a tradition. At this time everything stops for tea in England. People often come in for a chat with their cup of tea. Some English families like to have the so called "high tea". They have it between 5 and 6 o'clock.
      The main meal of the day is called dinner. Dinner is eaten either in the middle of the day or in the evening. The midday meal usually consists of two courses — a meat course with a lot of vegetables and a soup.

как найти Дискриминант (формула)

помогите пожайлуста с домашкой!!по математике у меня задача:Расстоение между двумя городами 1300км.какое расстояние между этими городами на карте,масштаб каторой 1:20000000???