9514 1404 393
Answer:
-7/4 < x < 8
Explanation:
The value of y can be determined from the sum of the angles, so you know each of the angles exactly. That means you know the ratio of side lengths exactly, which lets you solve for x exactly.
__
Setting that aside, we observe that angle C is greater than angle A, so side AB will be longer than side BC.
3x +15 > 4x +7
8 > x . . . . . . . . . . subtract 3x+7
We also know that the lengths of these sides must be positive. Since BC is the shorter side, we require ...
4x +7 > 0
4x > -7
x > -7/4
So, the allowable values of x are ...
-7/4 < x < 8
_____
More complete solution
If we read the figure correctly, the sum of angles is ...
(2y +12) +(4y +12) +(y -18) = 180
7y +6 = 180
y = (180 -6)/7 = 24 6/7°
Then (in degrees) ...
∠A = 2(24 6/7) +12 = 61 5/7, and ∠C = 4(24 6/7) +12 = 111 3/7
The Law of Sines tells us ...
AB/sin(C) = BC/sin(A)
sin(A)(3x +15) = sin(C)(4x+7)
x(4sin(C) -3sin(A)) = 15sin(A) -7sin(C)
x = (15sin(A) -7sin(C))/(4sin(C) -3sin(A))
x ≈ 6.1872652