not quite understanding this question

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find 2 positive numbers that satisfy the given requirements

1.) the sum of the first number and twice the second number is 108 and the product if a maximum

well....to me it seems as if i will be working with this equation

a+2b=108

a = 108-2b

put equation into all one term... i picked a

so.. 108-2b+2b=108

turns this into

108=108

rofl and no im lost how am i suppose to work this?