take the first equation and solve for one of the variables. The coefficient of variable (a) is one so it is easiest to solve for variable a rather than variable b.
a-2b = 3
a = 2b+3
Now bring in the second equation
3a = 6b + 9
In this equation we will substitute the value of a from the first equation. The first equation says a = 2b+3. So wherever you see the letter a, remove it and replace it with 2b+3
3(2b+3) = 6b + 9
Notice I put 2b+3 in parentheses. Whenever you have a quantity with two terms, it is safest to put parentheses around it, especially if you will be multiplying it as we will do here by the number 3.
6b + 9 = 6b + 9
0 = 0
Because 0 always equals 0, this system of equations has an infinite number of solutions.
