Let M ⟨u,v,w⟩ ∈ C uv ⊗C vw ⊗C wu denote the matrix multiplication tensor (and write M ⟨n⟩ = M ⟨n,n,n⟩ ) and let det3 ∈ (C 9 ) ⊗3 denote the determinant polynomial considered as a tensor. For a tensor T , let R(T ) denote its border rank. We (i) give the first hand-checkable algebraic proof that R(M ⟨2⟩ ) = 7, (ii) prove R(M ⟨223⟩ ) = 10, and R(M ⟨233⟩ ) = 14, where previously the only nontrivial matrix multiplication tensor whose border rank had been determined was M ⟨2⟩ , (iii) prove R(M ⟨3⟩ ) ≥ 17, (iv) prove R(det3) = 17, improving the previous lower bound of 12, (v) prove R(M ⟨2nn⟩ ) ≥ n 2 + 1.32n for all n ≥ 25 (previously only R(M ⟨2nn⟩ ) ≥ n 2 + 1 was known) as well as lower bounds for 4 ≤ n ≤ 25, and (vi) prove R(M ⟨3nn⟩ ) ≥ n 2 + 2n for all n ≥ 21, where previously only R(M ⟨3nn⟩ ) ≥ n 2 + 2 was known, as well as lower bounds for 4 ≤ n ≤ 21,.Our results utilize a new technique, called border apolarity developed by Buczyńska and Buczyński in the general context of toric varieties. We apply this technique to tensors with symmetry to obtain an algorithm that, given a tensor T with a large symmetry group and an integer r, in a finite number of steps, either outputs that there is no border rank r decomposition for T or produces a list of all potential border rank r decompositions in a natural normal form. The algorithm is based on algebraic geometry and representation theory. The two key ingredients are: (i) the use of a multi-graded ideal associated to a border rank r decomposition of any tensor, and (ii) the exploitation of the large symmetry group of T to restrict to B T -invariant ideals, where B T is a maximal solvable subgroup of the symmetry group of T .