Ok so let’s go stop by stop as we can see, a=3 and b=-12 right?
We won’t make it confusing so, we take the 4 and multiply 3 and 5, then it will be 12a+20+b, that’s 1st step!
So then the a and b will change to the following numbers on the equation, and everytime when a number has a letter beside it you always multiply like this: 12x3= 36, now it’s becomes 36+20+b, but we are not done yet, we still have the letter b, and it’s = to -12: 36+20+-12 : then the last step is just add them, 36+20=56, then take 56-12= 44, and for you’re information, if you’re confused a positive is the same thing as a normal number for example 10+ is the same exact thing as 10. I hope I helped you! Enjoy!