I'm so confused can someone me with number 4

Ok so first with a you have 
a=2 b=5 c=7
and the problems are 2a+10 and 2b+c
Well to do this we are going to right 2a+10 as 2*a+10
So now lets replace a.
and since a=2 we have 2*2+10
2*2=4+10=14
So A is 14
and now for be we have 2b+c
Lets re-write it as 2*b+c
and now replace
2*5+7
2*5=10+7=17
b is 17.
Now with the next (4)
For a it is x=7
Subtract 3x from both sides.
Subtract 5 from both sides.
x=7
and for B
x=3
Subtract 3x from both sides.
Add 10 to both sides.
Divide both sides by 4.
x=3
Hope this helps!

Wait what the question

Xis six meters can’t figure out b