You can solve this type of question two ways:
First cross multiply:
x = 3
42 7
(42)(3) = (7)(x)
This becomes :
126 = 7x
To solve for x divide both sides by 7 and you get x = 18
The other way to solve it is to think, what do I need to do to get x all by itself?
Since it's being divided by 42, you need to multiply each side of the equation by 42.
42 * x = 3 * 42
42 7
This leaves you with x = (42*3)/2
This simplifies to x = 18