Question 1065122
Numbers are  A and B  (let A be the larger number)

A = 2B+3   (1)
A+B = 21   (2)

Subs for A in (2):   (2B+3)+B = 21
                              3B + 3 = 21
                               3B = 18
                                 B = 6
                A=2B+3 = 2(6) + 3 = 12+3 = 15

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Ans:
The numbers are 6 and 15.
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Check:   6+15=21 (ok)
             2*6+3 = 12+3 = 15 (ok)