It's not so much about being good at math, it's more about practicing over and over. You can do it!
I hope this helps. :)
3/7 = a/21
Write the fraction in vertical form(I can't do it on here)with the 3 over the 7 and the a over the 21.
Draw a line from the 3 to the 21 by going through the equals sign.
Draw a line from the a to the 7 by going through the equals sign.
What you should see now is a big 'x' or cross. This is where the term 'cross multiplication' comes from, because now you are going to multiply 3(21) and 7(a).
Write an equation with the products of your multiplication equal to each other.
63 = 7a
Now, divide each side by 7.
Write the results of your division.
63 divided by 7 equals 9. 7a divided by 7 equals a.
9 = a is your answer.
To check your solution, put the value you found for a(9) in the original proportion and go through the same process.
3/7 = 9/21
3(21) = 7(9)
63 = 63 is a true statement, so your answer was correct!