Step-by-step explanation:
this is a little tricky because I am not entirely sure if the 2x+7 is all part of the exponent, but assuming it is, what you need to know is what value of x will make 2x + 7 = 15, that way each side will be
[tex] {2}^{15} [/tex]
[tex]2x + 7 = 15[/tex]
[tex]2x = 8[/tex]
[tex]x = 4[/tex]
