Question

if 27^15-3^y, what is the value of y ?

Answer 1

do you mean
27^15 = 3^y

?

if so, you might know that 3*3*3= 27
or you could say 27= \( 3^3 \)

replace 27 with \( 3^3 \)
\[ \left(3^3\right)^{15} = 3^y\]

now use the rule
\[ \left(a^b\right)^c = a^{bc} \]
to re-write the left side
can you finish ?