You need to be able to handle numbers of this sort without a calculator. Basically, you need to remember the following.
For multiplication, add exponents, and for division, subtract exponents.
The answer is 1.35x1014 (or rounded, 1.4x1014). In solving this, think: 3 x 5 = 15, and then add the exponents: 5 + 8 = 13. Move the decimal to the right of the first nonzero digit, or one place to the left.
Try another one: .
The answer is 3.4 x10 -12. In solving this, think: 6.8/2 = 3.4, and then subtract the exponents: (-2) - (10) = -12.