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  60

Step-by-step explanation:

In the function, the growth factor is "a", the base of the exponent. When the exponent is increased by 1, the value is multiplied by "a".

  f(7) = a·f(6) = 4·15

  f(7) = 60

As you know, the exponent signifies repeated multiplication. So, an increase of 1 in the exponent means the base is part of the product one more time:

  a^6 = a·a·a·a·a·a

  a^7 = a·a·a·a·a·a·a = a·(a^6)