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For what values of the variable x is the expression square root of 7x+14+ square root of 7-x a real number?

Ebuka's monthly rent is $750. if ebuka pays the rent late, his landlord charges 4% interest per week that the payment is late. write a function that gives the total cost r(t), in dollars, of ebuka's rent if he pays it t weeks late.

Math i have no idea how to do a and b