You would work on the order of operations PEDMAS
P= Parenthasis
E= Exponets
D/M= Division/Multiplication
A/S= Addition/Subtraction
D/M carry the same weight so you do them left to right, the same with A/S you do them left to right as well.
So lets look at your problem
7+9*3-1 there are no parenthasis so we can look at exponets. There are no exponents so now we look for D/M
7+9 no that isn't division or multiplication
9*3 that is multiplication so we do that now 9*3= 27
so we now have 7+27-1, there is no more division or multiplication so we move to addition and subtraction working left to right
7+27=34 so now we have 34-1=33
so 7+9*3-1=33