The expression 5x+3 would be three more than five times a number! More than in this case would be addition - so if you get the same problem with subtraction you just change more to less!
Basically what it's tell you is that if x stood for 2 in this case 5*2 = 10 since it's 3 MORE than 5 times a number (x is in place of a number), you're adding 3 to 10 and that gives you 13!
I hope I simplified it a little more and you understand!
Good luck, rockstar! c: I hope this helped!