let x be the base the altitude will be x-7
Now as per pythagorus theorem.
[tex] {x}^{2} + (x - 7 {)}^{2} = 1 {3}^{2} \\ {x}^{2} - 7x - 69 = 0 \\ \times = 12 \: \: or \: \: - 5 \\ [/tex]
Since x cannot be negative
[tex] \times = 12[/tex]
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