If you take a number, times by 5 then add 7. You get the same as if you took the number, times by 9 then add 3. What is the number?

Iam trying to solve a linear equation for the shoe size of someone 84" tall taking into consideration example 1 is 69" tall with a shoe size of 9 and example 2 is 72 inches tall with a shoe size of 11. i understand that the equation should be y=mx+b and that on the graph the answer is (3,2) but i just don't understand how to create the equation to solve for 84-inches and 60-inches tall questions. after two days, i need .

Find the solution of this system of equation -7x+y=-20 9x-3y=36

Find the area of the regular hexagon if a side is 20 cm.