Seven less, means that you subtract 7.
The product of three and a number, means you multiply 3 and x where x=the unknown number.
Is -16 means that is is equal to -16.
Let's put it all together.
3x - 7 = -16
If you want to solve for x:
3x - 7 = -16
Add 7 to both sides.
3x = -9
Divide 3 on both sides.
x = -3
Hope this helps!