Yes you’re right you cannot know if the equation is true or false
With equations, you can add, subtract, multiply and/or divide by any amount so long as whatever you do is done to both sides of the equation;
Using this principle we can rearrange equations to give us the value of an unkown variable.
Y= 3x + 1
If x is 2 we plug in 2 instead of x
The equation becomes y= 3(2) + 1
now you just solve
y= 6+1
y=7