Let y be the number.
3 less than 2 a number.
2 a number - 3
2y - 3
is the algebraic expression for this, you can make the variable (y) another (x, m ,n, etc.)
Less Than says we have to lessen, or subtract, means that we should make it a (-)
And "a certain number" should be a "variable" and that is any variable, in this case it's y.