"Eighteen is..."
The "is" is the equals sign:
18 = ...
"Eighteen is fifteen less than..."
"..fifteen less than..." indicates the 15 is going to be subtracted from something; we don't yet know what that something is:
18 = ??? - 15
"Eighteen is fifteen less than the product of a number and three."
Now we know what the fifteen is being subtracted from -- the product of a number and 3:
18 = (x)(3) - 15
That is how the mathematical statement looks if we translate the words as directly as possible. In the usual mathematical form the statement would be
18 = 3x - 15
Solve using basic algebraic steps.
Add 15 to both sides to get the x term alone:
18+15 = 3x-15+15
33 = 3x
Divide both sides by 3 to get x alone:
33/3 = 3x/3
11 = x
