You want to follow the order of operations:
do things inside grouping/parentheses first
then do any exponents
then do multiplication/division, left-to-right
then do addition/subtration, left-to-right
15 - 4 x 3: do multiplication before subtration
15 - 4 x 3 = 15 - 12 = 3
5 + ( 2 + 1 ) ^ 3: do what's in parentheses, then exponent, then addition
5 + (2+1) ^ 3 = 5 + 3^3 = 5+27 = 32
7 + 4 x 2^3: do exponent first, then multiplication, then addition:
7 + 4 x 2^3 = 7 + 4 x 8 = 7 + 32 = 39
