I need to solve a system of equations.

Everything works until the part where I want to solve for the coefficients which I have stored in the vector a:

Is there a way to solve for these coefficients?

My script is the following:

```
using SymPy
using Symbolics
@variables a₁ a₂ a₃ a₄ L x
a = [a₁,a₂,a₃,a₄]
phi = 0;
for i in axes(a,1)
phi = phi + a[i]*x^(i-1)
end
phid = Symbolics.derivative(phi, x)
# Defining the equations
equ1 = [substitute(phi, x => 0) => 1, # phi(0)=1
substitute(phi, x => L) => 0, # phi(L)=0
substitute(phid, x => 0) => 0, # dphi(0)=0
substitute(phid, x => L) => 0] # dphi(L)=0
# The part that does not work!!::
# Solve for a₁ a₂ a₃ a₄
# sol1 = solve(equ1, a)
# sol1.a₁
# sol1.a₂
# sol1.a₃
# sol1.a₄
# Next step
# Substitute coefficients into phi
phi_t1 = substitute(phi, a => [sol1.a₁,sol1.a₂,sol1.a₃,sol1.a₄])
```