Because both lower inside angles are 30 degrees, the 2 angles sides are the same length.
Set those to equal to solve for X, then use the value of X to solve for the lengths of all 3 sides.
42 +X = 2x-4
Add 4 to each side:
46 + x = 2x
Subtract 1X from each side:
46 = X
Now replace X with 46 in each equation:
42 + x = 42 + 46 = 88
2x-4 = 2(46) - 4 = 92-4 = 88
3x +10 = 3(46) +10 = 138 +10 = 148
Perimeter = 148 + 88 + 88 = 324 units