Step-by-step explanation: Since you know this triangle is an equilateral, you know all angles are the same, so you should automatically multiply 10x+20 by 3. You would then get 30x+60. Since all triangle's total angle sum is 180, you know to set 30x+60 to 180.
At this point this might seem new, but this is where you isolate x. First you move the 60, but subtracting it from both sides. This gets you 30x=120. You divide 30 on both sides and you finally get your answer: 4.