Follow the order of operations to find the unknown number 6+4 x 3

You need to multiply first, so 4x3=12, then add the 6 to equal 18.

In this case, we follow BODMAS. 
(B- Bracket, O- Of, D- Division, M- Multiplication, A- Addition, S- Subtraction)
Because Multiplication comes first, we multiply 4 and 3. Leaving our sum to
6+12 which is equal to 18.
∴ 6+4 x 3= 18