First, rewrite this expression as:
#(a^4/a)(b^-3/b^-2)#
Use these rules for exponents to simplify the #a# terms:
#a = a^color(blue)(1)# and #x^color(red)(a)/x^color(blue)(b) = x^(color(red)(a)-color(blue)(b))#
#(a^4/a)(b^-3/b^-2) => (a^color(red)(4)/a^color(blue)(1))(b^-3/b^-2) => a^(color(red)(4)-color(blue)(1))(b^-3/b^-2) =>#
#a^3(b^-3/b^-2)#
Next, use these rules of exponents to simplify the #b# terms:
#x^color(red)(a)/x^color(blue)(b) = 1/x^(color(blue)(b)-color(red)(a))# and #a^color(red)(1) = a#
#a^3(b^color(red)(-3)/b^color(blue)(-2)) => a^3(1/b^(color(blue)(-2)-color(red)(-3))) => a^3(1/b^(color(blue)(-2)+color(red)(3))) =>#
#a^3(1/b^color(red)(1)) =>#
#a^3/b#