-2
3
4
1

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If the line is tangent to the cubic at $x= 1$ , then both must intersect at that point.

The y-value on the line at $x=1$ is: $\left( -3 \right) (1)+4=1$

The y-value on the curve at $x = 1$ is: ${ 1 }^{ 3 }(a)+(1)b=a+b$

Since both y-values must be the same since the line is tangent to the curve, $a + b = 1$