`a,`
`3. (x + 7) = 21
`=> x + 7 = 21 : 3`
`=> x + 7 = 7`
`=> x = 7 - 7`
`=> x = 0`
Vậy `x = 0`
`b,`
`2x + 1/4 = 3/2`
`=> 2x = 3/2 - 1/4`
`=> 2x = 5/4`
`=> x = 5/4 : 2`
`=> x = 5/4 . 1/2`
`=> x = 5/8`
Vậy `x = 5/8`
`c,`
`5^(2x - 3) - 50 = 5^2 . 3`
`=> 5^(2x - 3) - 50 = 75`
`=> 5^(2x - 3) = 75 + 50`
`=> 5^(2x - 3) = 125`
`=> 5^(2x - 3) = 5^3`
`=> 2x - 3 = 3`
`=> 2x = 3+3`
`=> 2x = 6`
`=> x = 3`
Vậy `x = 3`
`d,`
`x - 1,5 = (2x)/3`
`=> 3 . (x - 1,5) : 3 = (2x)/3`
`=> 3x - 4,5 : 3 = (2x)/3`
`=> 3x - 4,5 = 2x`
`=> 3x - 2x = 4,5`
`=> x = 4,5`
Vậy `x = 4,5`
