When I go faster than 75 km h  my four-year-old car starts to shake  Even the backseat passengers can feel it. A friend told me that it could be because of the mounting or pulley wheels. What is the likely fault?

There are few possibilities. Worn wheel bearings could be one. Or misaligned wheels. Or unbalanced tires  The most serious would be damaged  suspension parts. Have your car checked by a competent mechanic as soon as possible.