(3a)
Note that angle CBD = angle CBA
And angle CDB = angle BCA
So....by AA congruency, triangle BDC is similar to triangle BCA
(3b)
Note that angle CAB = angle CAD
And angle ACB = angle CDA
So....by AA congruency, triangle ADC is similar to triangle ACB
For the second one....it's easier to find y first
We have that
12/5 = y/3 cross- multiply...
3*12 = 5*y
36 = 5y divide both sides by 5
36/5 = y = 7.2
And
[12 + 5 ] / x = [y + 3 ] / 6
[ 17 ] / x = [ 7.2 + 3 ] / 6 cross-multiply
17*6 = 10.2 x
102 = 10.2 x divide both sides by 10.2
10 = x