Answer:
[tex]\dfrac{a + 3}{3}[/tex]
Step-by-step explanation:
[tex]\begin{array}{rcl}5a + 7x & = & 3(2a +1) + 4x\\5a + 7x & = & 6a +3 + 4x\\7x & = & a + 3 + 4x\\3x & = & a + 3\\x & = & \mathbf{\dfrac{a + 3}{3}}\\\end{array}[/tex]
