Answer:
38 degrees
Explanation:
Vertical angles are congruent(equal measures), so mLA = mLB
STEP 1:
Let's use some simple substitution.
mLA = mLB
mLA = x+21, mLB = 4x-30
You plug these two in and get:
x+21 = 4x-30
This is your equation.
STEP 2:
Let's solve our equation!
x+21 = 4x-30
(add 30 to both sides)
x+51 = 4x
(subtract x from both sides)
51 = 3x
(switch order for comprehension)
3x = 51
(divide both sides by 3)
x = 17
Ta-da! You get the measure of x = 17 degrees.
STEP 3:
Let's plug in our value of x to get the value of LB.
mLB = 4x - 30
mLB = 4(17) - 30
mLB = 68 - 30
mLB = 38
This is your answer.