Most solvers available on the market do not accept the differential equation in this form--
the second-order differential equation, which is not an explicit form.
What you instead have to provide is a function that returns the first derivative, the rate of change.
The trick is to turn this differential equation into two differential equations of first order.
Here is a hint for you--the first equation should be that the derivative of x with respect to time
is some new variable called y, and now you have to provide the second equation
for the derivative of y with respect to time.
Should that be 7-3*y+4*x or 7-3 times the derivative of x plus 4x
or 1/3*7 minus the second derivative plus 4*x or 1/3*7 minus the first derivative of y plus 4*x.