Find the root

.text

.global solve

solve:
        push {r4 - r7}
        /// r4 will be our x variable
        mov r4, #-1

        start:
                add r4, #1

                /// C * x
                mul r7, r2, r4

                /// B * X^2
                mul r6, r1, r4
                mul r6, r4

                /// A * x^3
                mul r5, r0, r4
                mul r5, r4
                mul r5, r4

                /// A * X^3 + B * X^2 + C * X + D
                add r5, r6
                add r5, r7
                add r5, r3

                /// X(Ax^3 + Bx^2 + Cx + D)
                mul r5, r4

                /// Add constant E
                ldr r7, [sp, #16]
                add r5, r7

                cmp r5, #0
                bne start

        mov r0, r4

        pop {r4 - r7}
        bx lr