MORE PHYSICAL SCIENCE HOMEWOK HELP?

The key is to look at the units, and remember that Hz is the same as 1/s. Also, I think your speed on the first problem should be 3*10^8 (m/s) instead of 3*10^6 (m/s). For one thing, 3*10^8 is the speed of light assuming it's not slowed down by traveling through anything like water, and for another thing a speed of 3*10^6 doesn't give one of the listed answers.

The main equation for these two problems is: wavelength * frequency = speed.

1. I always put my units in parentheses. 3.0*10^8 (m/s) / 1.5*10^-7 (m) = 2*10^15 (1/s) = 2*10^13 (Hz). The meters cancel out, and give you 1/s which is Hz.

2. 3*10^8 (m/s) / 0.032 (m) = 9.375*10^9 (1/s) = 9.375*10^9 (Hz).  Same thing as the last problem, just different numbers.

Since you were stuck, I'm guessing that what you're having trouble with is understanding the way the units make the equation work (or vice versa, however you want to say it). Units cancel just like numbers. If you got an answer in meters, or in watts, you'd know you did something wrong because that's not the units for frequency. If you can make meters cancel like I did and get frequency, you've most likely used the right equation, so you don't even have to memorize that equation as long as they give you the units in the problem (and you remember what Hz really is).