Without a calculator how would I calculate multiplication exponential expression

The formula is a^x a^ y = a ^x+ y

(7 x 10^ 2) x (4 x 10 ^ 5)

i thought the answer would be 28 x 10^ 7 but the answer key has 2.8 x 10 ^ 8

so what am I doing wrong?

in the similar way, (5 x 10 ^ -3) x ( 4 x 10^ -6)

i thought the answer would be 20 x 10 ^ -8 but the answer is 2 x 10 ^-8

Thank you