First off, a triangle will be 180 all together, so, x + 59 + x + 51 = 180, 59 + 51 is 110, x + x is 2x, 180 - 110 is 70 so you have 2x = 70, now divide both sides by two, The left will cancel out to x and the other will be 35, x = 35, now you are looking for angle a, in order to find this, you have to plug in your X, so 51+35 is 86, that should be your answer, I hope this helps!