Other guy didn’t explain so here it is
The two angles are equal as they are vertically opposite. So, we will show that algebraically as
5x-30 = 4x
Now we solve the equation.
Subtract 4x from both sides to get 5x-30-4x=0
Now add 30 to both sides to get 5x-4x=30
Now do 5x-4x=x
X=30