Answer:
x = 6
explanation:
USE: [tex]\boxed{\frac{side-length }{base -length} = \frac{side-length}{base -length} }[/tex]
solve:
[tex]\frac{21}{15} = \frac{7}{x-1}[/tex]
[tex]21(x-1) = 7(15)[/tex]
[tex]21x-21 = 105[/tex]
[tex]21x = 105+21[/tex]
[tex]21x = 126[/tex]
[tex]x = \frac{126}{21}[/tex]
[tex]x = 6[/tex]
