If a question involves both multiplication and addition then the convention is that the multiplication should be carried out first, then the addition.
So for 4 + 5 x 3
Firstly multiply 5 by 3 and then add on 4 to make 19.
Now some calculators do this (Scientific etc) but some work out each part as it is keyed in. This results in two calculators arriving at different answers: always interesting to show children!