Find the difference do not round 17.216 - 4.63

Tomas is making trail mix using granola and walnuts. he can spend a total of $12 on the ingredients. he buys 3 pounds of granola that costs $2.00 per pound. the walnuts cost $6 per pound. he uses the equation to represent the total cost, where x represents the number of pounds of granola and y represents the number of pounds of walnuts. he solves the equation for y, the number of pounds of walnuts he can buy. answer- b tomas added 6 to both sides of the equation instead of subtracting 6. just took the test

Xy x xy if a and b are positive integers such that a*b=9 and b*a=72 what is the value of a/b

How are algebraic expressions evaluated using substitution?