Note
Tangents from an external point to a circle's circumference are equal.
So
WZ = WX
WZ = 5a + 12
WX = 7a + 5
7a + 5 = 5a + 12 Subtract 5 from both sides.
7a = 5a + 12 - 5
7a = 5a + 7 Subtract 5a from both sides.
7a - 5a = 7
2a = 7 Divide by 2
a = 7/2
a = 3.5
