First group the coefficients and variables.

(5r³)(4r⁴) = ( 5 · 4) (r³ · r⁴)

=  (20) (r³ · r⁴)            (Multiply: 5 · 4 = 20)

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

=  (20) (r^{3 + 4})

=  20r⁷                        (Add exponents: 3 + 4 = 7)

Product of  (5r³)(4r⁴) is 20r⁷.