'The number' should be represented by an 'x' so you have to write 12 less than 2x is equal to 15 minus 7 times x
The 15 minus 7 times x part is not that difficult, you have to write something like 3y for the 7 times x part and that goes after the 15 and the minus sign.
the '12 less than 2x' is basically just saying x minus 12.
Hope this helps :)