book Maths Puzzle Numerical Ability

Can you explain the functions of x better to me? Just started Class and I am not understanding it

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book Other Question

The two biggest lakes in Manitoba are lake Winninpeg and Lake Brandon. Their combined area is about 29800 km^2. The area of lake Brandon is 700 km^2 less than one quarter of the area of lake Winnipeg. What is the area of each lake? the preimeter of a triangle is given by the formula P=? (thank you for the help)