First group the coefficients and variables.

2y(9y⁴) = ( 2 · 9) (y · y⁴)

=  ( 18) (y · y⁴)            (Multiply:   2 · 9 =  18)

Since base is common, apply the product of powers property: a^m · aⁿ = a^{m + n}.

=  (18) (y^{1 + 4})

=  18y⁵                        (Add exponents: 1 + 4 = 5)

Product of  2y(9y⁴) is 18y⁵.