`a)` `7^x . 49 = 7^2021`
`7^x = 7^2021 : 7^2`
`7^x = 7^2019`
`=>` `x = 2019`
`b)` `2^x+1 . 3 + 15 = 39`
`2^x+1 . 3 = 24`
`2^x+1 = 8`
`x + 1 = 4`
`x = 4 - 1 = 3`
`c)` `2^x+7 = 2^2 . 16 + 192`
`2^x+7 = 256`
`2^x+7 = 2^8`
`=>` `x = 1`
`d)` `102 : [(2^x + 13) - 4] = 6`
`(2^x + 13) - 4 = 17`
`2^x + 13 = 21`
`2^x = 8`
`x = 3`