In the above line:
∠y + 56 + 51 = 180
To find y, simplify, isolate and simplify again to get y. Remember, what you do to one side, you do to the other.
∠y + 107 = 180
∠y + 107 (-107) = 180 (-107)
∠y = 180 - 107
∠y = 73°
A triangle is = 180°. With this knowledge, you can find ∠x (the last angle).
Plug in 73° for y
y = 73
73 + 72 + x = 180
Simplify.
x + 145 = 180
isolate the x, subtract 145 from both sides
x + 145 (-145) = 180 (-145)
x = 180 - 145
x = 35°
A) 35° is your answer
hope this helps