You just need to know the order of operations or PEMDAS. To evaluate the expressions, first, do anything in parentheses, then exponents, then multiplication and division, then addition and subtraction.
a) 6x7-3^2x9+4^3
6x7-9x9+64
42-81+64
25
b) 5+4x2+6-2x2-1
5+8+6-4-1
14
