You started answering this question correctly: by simplifying the equation. Using the simplified equation that you came up with, we can find the value of a and b where there is an infinite number of solutions. There is an infinite number of solutions when the left side matches the right side (because the two lines will be the same). So, the answer to the first part is a= 30 and b=-48.
To solve the second part, start by plugging in 10 and -4 for a and b. You will end up with: 30x-48=10x-4.
The first thing to do is to subtract 10x from both sides so you can get the x on one side of the equation. The equation becomes: 20x-48=-4.
Now, you can solve this problem like any other two-step linear function. Add 48 to both sides:
20x=44
Divide both sides by 20:
x=44/20
44/20 can be simplified by dividing the numerator and the denominator by the GCF (which is 4). It becomes:
11/5
Hope this answers your question!