The smaller triangle has a ratio of 11 : 10.
The smaller triangle and the larger triangle should have the same ratio.
The total of the left side of the larger triangle is 132cm
The ratio of the larger triangle is:
132 : (5x + 10) + 10
132 = 5x + 20
If you multiply the 11 : 10 ratio by 12, you get:
132 : 120
Since you know the other side should equal 120, to find x you do:
5x + 20 = 120
5x = 100
x = 20