`a,`
`(3x + 2)^2 - 21 = 4`
`=> (3x + 2)^2 = 4 + 21`
`=> (3x + 2)^2 = 25`
`=>` \(\left[ \begin{array}{l}(3x + 2)^2 = 5^2\\(3x + 2)^2 = (-5)^2\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}3x + 2 = 5\\3x + 2 = -5\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x = 1\\x=\dfrac{-7}{3}\end{array} \right.\)
`b,`
`3^(x + 2) + 3^x = 810`
`=> 3^x . 9 + 3^x = 810`
`=> 3^x . (1 + 9) = 810`
`=> 3^x . 10 = 810`
`=> 3^x = 81`
`=> x = 4`
`c,`
`1440 : [120 - (3x + 9)] = 120`
`=> 120 - (3x + 9) = 1440 : 120`
`=> 120 - 3x - 9 = 12`
`=> 111 - 3x = 12`
`=> 3x = 111 - 12`
`=> 3x = 99`
`=> x = 33`
