Write the problem out like this:
7 x 3
45 x 2
It's easier to see what you're doing. Look at the numbers on the top (numerator) and see if any of them divide into numbers at the bottom (denominator), or if there's a common factor between a number on top with a number on the bottom. In this case, 3 goes into 45 15 times so we mark the fraction:
7 x 3
45 x 2
15
The fraction has now become:
7
15 x 2
Do the multiplication on the bottom 15 x 2 = 30 so we now have 7/30:
7
30